Work around GCC 13's bogus "out of bounds" check to QByteArray::_empty

Nothing here is empty and even if anything were, QStringBuilder properly
handles empty strings.

In static member function ‘static void QConcatenable<QByteArrayView>::appendTo(QByteArrayView, char*&)’,
    inlined from ‘static void QConcatenable<QStringBuilder< <template-parameter-1-1>, <template-parameter-1-2> > >::appendTo(const type&, T*&) [with T = char; A = QByteArrayView; B = const char (&)[20]]’ at qstringbuilder.h:398:37,
    inlined from ‘T QStringBuilder< <template-parameter-1-1>, <template-parameter-1-2> >::convertTo() const [with T = QByteArray; A = QByteArrayView; B = const char (&)[20]]’ at qstringbuilder.h:117:54,
    ...
qstringbuilder.h:178:19: error: ‘void* memcpy(void*, const void*, size_t)’ forming offset [1, 5] is out of the bounds [0, 1] of object ‘QByteArray::_empty’ with type ‘const char’ [-Werror=array-bounds=]

QStringBuilder::convertTo() creates the target as
        const qsizetype len = QConcatenable< QStringBuilder<A, B> >::size(*this);
        T s(len, Qt::Uninitialized);

We know len can't be zero because GCC is complaining about a memcpy()
when the offset has been changed from 0, meaning QByteArray was given a
non-zero size and therefore its data pointer is not &QByteArray::_empty.

Fixes: QTBUG-116763
Pick-to: 6.5 6.6
Change-Id: I85599ea5ca7a4b79a8bbfffd178af437984080fb
Reviewed-by: Shawn Rutledge (away) <shawn.rutledge@qt.io>

src/gui/image/qimagereaderwriterhelpers.cpp

@@ -99,6 +99,8 @@ QList<QByteArray> supportedImageFormats(Capability cap)
 
 static constexpr QByteArrayView imagePrefix() noexcept { return "image/"; }
 
+QT_WARNING_PUSH
+QT_WARNING_DISABLE_GCC("-Warray-bounds") // QTBUG-116763
 QList<QByteArray> supportedMimeTypes(Capability cap)
 {
     QList<QByteArray> mimeTypes;
@@ -114,6 +116,7 @@ QList<QByteArray> supportedMimeTypes(Capability cap)
     mimeTypes.erase(std::unique(mimeTypes.begin(), mimeTypes.end()), mimeTypes.end());
     return mimeTypes;
 }
+QT_WARNING_POP
 
 QList<QByteArray> imageFormatsForMimeType(QByteArrayView mimeType, Capability cap)
 {