QtCore: Don't compare QChars to literal 0s

This is prone to ambiguities, even though we currently don't run
into them.

Use QChar::isNull() instead.

Change-Id: I71843878b3f4f8a5deae2ef57a6f6628461be216
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/io/qdir.cpp

@@ -138,7 +138,7 @@ inline QChar QDirPrivate::getFilterSepChar(const QString &nameFilter)
 // static
 inline QStringList QDirPrivate::splitFilters(const QString &nameFilter, QChar sep)
 {
-    if (sep == 0)
+    if (sep.isNull())
         sep = getFilterSepChar(nameFilter);
     QStringList ret = nameFilter.split(sep);
     for (int i = 0; i < ret.count(); ++i)

src/corelib/tools/qstring.cpp

@@ -1509,7 +1509,7 @@ QString::QString(const QChar *unicode, int size)
     } else {
         if (size < 0) {
             size = 0;
-            while (unicode[size] != 0)
+            while (!unicode[size].isNull())
                 ++size;
         }
         if (!size) {