vcl: avoid OUString defaulted parameter in GDIMetaFile::dumpAsXml()

So that it's possible to do ad-hoc dumping from gdb with something like:

print xMetaFile->dumpAsXml(0)

Similar to how that already worked for SwDoc or SwFrame.

Change-Id: I9cb884491f2a49afdb06179d8d54252101f37bb7
Reviewed-on: https://gerrit.libreoffice.org/69253Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins

include/vcl/gdimtf.hxx

@@ -199,7 +199,7 @@ public:
     bool            GetUseCanvas() const { return m_bUseCanvas; }
 
     /// Dumps the meta actions as XML in metafile.xml.
-    void dumpAsXml( const OUString& sFileName = OUString() ) const;
+    void dumpAsXml(const char* pFileName = nullptr) const;
 };
 
 #endif // INCLUDED_VCL_GDIMTF_HXX

vcl/source/gdi/gdimtf.cxx

@@ -2835,9 +2835,10 @@ void GDIMetaFile::UseCanvas( bool _bUseCanvas )
     m_bUseCanvas = _bUseCanvas;
 }
 
-void GDIMetaFile::dumpAsXml( const OUString& sFileName ) const
+void GDIMetaFile::dumpAsXml(const char* pFileName) const
 {
-    SvFileStream aStream(sFileName.isEmpty() ? "metafile.xml" : sFileName, StreamMode::STD_READWRITE | StreamMode::TRUNC);
+    SvFileStream aStream(pFileName ? OUString::fromUtf8(pFileName) : OUString("metafile.xml"),
+            StreamMode::STD_READWRITE | StreamMode::TRUNC);
     MetafileXmlDump aDumper;
     aDumper.dump(*this, aStream);
 }