Cleanup. Don't return NULL when RString is expected.

stepmania/src/ThemeManager.cpp

@@ -591,7 +591,7 @@ try_element_again:
 	
 
 	if( asElementPaths.size() == 0 )
-		return NULL;	// This isn't fatal.
+		return RString();	// This isn't fatal.
 
 	FilterFileLanguages( asElementPaths );
 
@@ -725,7 +725,7 @@ try_element_again:
 	if( bOptional )
 	{
 		Cache[sFileName] = "";
-		return NULL;
+		return RString();
 	}
 
 	const RString &sCategory = ElementCategoryToString(category);
@@ -860,7 +860,7 @@ RString ThemeManager::GetMetricRaw( const IniFile &ini, const RString &sClassNam
 				LOG->UserLog( RageLog::LogType_ThemeMetric, sClassName + " : " + sValueName,
 					      "could not be found in \"%s\" or \"%s\".",
 					      sCurMetricPath.c_str(), sDefaultMetricPath.c_str() );
-				return NULL;
+				return RString();
 			default:
 				ASSERT(0);
 		}