Replace confusing member variable with a predicate

The Converter class, in the eponymous example, had a null member
variable that wasn't a nullptr - it pointed to an instance of
NullConverter - so that other converters could test whether a
Converter * they'd been passed was null (in the sense of pointing to a
NullConverter). This, however, was susceptible to misreading - I
misread one such comparison as a nullptr check and thus thought it
redundant with an earlier actual nullptr check. To spare future
readers similar confusion, replace the public static member variable
with a protected (since only other derived classes need it) static
predicate, to at least give the reader a clue that this is using the
word null in a class-specific sense.

Pick-to: 6.6 6.5
Task-number: QTBUG-111228
Change-Id: I1e4f494b303d1bf90107f8c6fa3a4a22f6d81b90
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

examples/corelib/serialization/convert/cborconverter.cpp

@@ -250,7 +250,7 @@ QVariant CborConverter::loadFile(QIODevice *f, const Converter *&outputConverter
 
     if (outputConverter == nullptr)
         outputConverter = &cborDiagnosticDumper;
-    else if (outputConverter == null)
+    else if (isNull(outputConverter))
         return QVariant();
     else if (!outputConverter->outputOptions().testFlag(SupportsArbitraryMapKeys))
         return contents.toVariant();

examples/corelib/serialization/convert/converter.h

@@ -12,9 +12,9 @@ class Converter
 {
 protected:
     Converter();
+    static bool isNull(const Converter *converter); // in nullconverter.cpp
 
 public:
-    static Converter *null;
 
     enum class Direction { In = 1, Out = 2, InOut = In | Out };
     Q_DECLARE_FLAGS(Directions, Direction)

examples/corelib/serialization/convert/jsonconverter.cpp

@@ -76,7 +76,7 @@ QVariant JsonConverter::loadFile(QIODevice *f, const Converter *&outputConverter
         qFatal("Could not parse JSON content: offset %d: %s",
                error.offset, qPrintable(error.errorString()));
     }
-    if (outputConverter == null)
+    if (isNull(outputConverter))
         return QVariant();
     return doc.toVariant();
 }

examples/corelib/serialization/convert/nullconverter.cpp

@@ -6,7 +6,10 @@
 using namespace Qt::StringLiterals;
 
 static NullConverter nullConverter;
-Converter *Converter::null = &nullConverter;
+bool Converter::isNull(const Converter *converter)
+{
+    return converter == &nullConverter;
+}
 
 QString NullConverter::name() const
 {