STYLE: default construct empty dictionaries

- some code used copy construct from dictionary::null instead.
  The result is the same but suggests that something else may be intended.
  Only need dictionary::null for const-ref usage.

applications/utilities/miscellaneous/foamHelp/helpTypes/doxygenXmlParser/doxygenXmlParser.C

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2012-2016 OpenFOAM Foundation
+    Copyright (C) 2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -39,7 +40,7 @@ Foam::doxygenXmlParser::doxygenXmlParser
     const word& ext
 )
 :
-    dictionary(dictionary::null)
+    dictionary()
 {
     // Pre-construct and compile regular expressions
     const regExp nameRe(".*.H");
@@ -155,7 +156,7 @@ Foam::doxygenXmlParser::doxygenXmlParser
                     //    preferentially take exact match if it exists
                     if (exactMatch && (tName + "." + ext) == name)
                     {
-                        dictionary dict(dictionary::null);
+                        dictionary dict;
                         dict.add("name", name);
                         dict.add("filename", fName + ".html");
                         dict.add("path", path);
@@ -168,7 +169,7 @@ Foam::doxygenXmlParser::doxygenXmlParser
                      && regExp(".*" + tName + ".*").match(name)
                     )
                     {
-                        dictionary dict(dictionary::null);
+                        dictionary dict;
                         dict.add("name", name);
                         dict.add("filename", fName + ".html");
                         dict.add("path", path);

applications/utilities/miscellaneous/foamHelp/helpTypes/doxygenXmlParser/doxygenXmlParser.H

@@ -53,7 +53,6 @@ class doxygenXmlParser
 :
     public dictionary
 {
-
 public:
 
     //- Construct from components

applications/utilities/preProcessing/createZeroDirectory/boundaryTemplates.C

@@ -40,8 +40,8 @@ Foam::boundaryTemplates::boundaryTemplates
     const word& solverType
 )
 :
-    templates_(dictionary::null),
-    options_(dictionary::null)
+    templates_(),
+    options_()
 {
     Info<< "    Reading boundary templates" << endl;
 
@@ -62,8 +62,8 @@ Foam::boundaryTemplates::boundaryTemplates
         const word& regionType = dEntry.keyword();
         wordList patchTypes(regionBCs.lookup(regionType));
 
-        dictionary regionTemplate = dictionary::null;
-        dictionary regionOptions = dictionary::null;
+        dictionary regionTemplate;
+        dictionary regionOptions;
 
         // read general boundary types
         forAll(patchTypes, i)
@@ -250,8 +250,8 @@ Foam::dictionary Foam::boundaryTemplates::generatePatchDict
             // look for field name
             if (patchDict.found(fieldName))
             {
-                dictionary dict(dictionary::null);
-                const dictionary& fieldDict(patchDict.subDict(fieldName));
+                dictionary dict;
+                const dictionary& fieldDict = patchDict.subDict(fieldName);
 
                 for (const entry& dEntry : fieldDict)
                 {

applications/utilities/preProcessing/createZeroDirectory/caseInfo.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2015 OpenFOAM Foundation
-    Copyright (C) 2015 OpenCFD Ltd.
+    Copyright (C) 2015-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -194,7 +194,7 @@ Foam::dictionary Foam::caseInfo::generateBoundaryField
     const boundaryTemplates& bcTemplates
 ) const
 {
-    dictionary boundaryField = dictionary::null;
+    dictionary boundaryField;
 
     forAll(boundaryInfo_.names(), j)
     {
@@ -202,7 +202,7 @@ Foam::dictionary Foam::caseInfo::generateBoundaryField
 
         if (boundaryInfo_.constraint()[j])
         {
-            dictionary patchDict = dictionary::null;
+            dictionary patchDict;
             patchDict.add("type", boundaryInfo_.types()[j]);
 
             // Add value for processor patches
@@ -228,7 +228,7 @@ Foam::dictionary Foam::caseInfo::generateBoundaryField
 
             const word& patchType = patchTypes_[conditionI];
 
-            dictionary optionDict = dictionary::null;
+            dictionary optionDict;
             if (bcTemplates.optionsRequired(regionPrefix, category, patchType))
             {
                 optionDict = bcDict_.subDict(condition).subDict("options");