Avoid deref on missing part

We do not always have the LIB_SYMBOL set in a SCH_SYMBOL, so avoid
trying to get the embedded files from it

Fixes https://gitlab.com/kicad/code/kicad/-/issues/20986

eeschema/sch_symbol.cpp

@@ -1315,7 +1315,9 @@ bool SCH_SYMBOL::ResolveTextVar( const SCH_SHEET_PATH* aPath, wxString* token, i
 
         std::vector<EMBEDDED_FILES*> embeddedFilesStack;
         embeddedFilesStack.push_back( schematic->GetEmbeddedFiles() );
-        embeddedFilesStack.push_back( GetLibSymbolRef()->GetEmbeddedFiles() );
+
+        if( m_part )
+            embeddedFilesStack.push_back( m_part->GetEmbeddedFiles() );
 
         simLibMgr.SetFilesStack( embeddedFilesStack );