ALL: Reduce assignment of "" to Common::String.

When clearing an existant object clear() should be used.

When constructing objects (or using default values for parameters) the
constructor of String without any argument should be used.

This changes only a few instances I noticed while looking over some recent
commit logs.

audio/mididrv.cpp

@@ -74,7 +74,7 @@ static const struct {
 };
 
 Common::String MidiDriver::musicType2GUIO(uint32 musicType) {
-	Common::String res = "";
+	Common::String res;
 
 	for (int i = 0; GUIOMapping[i].guio; i++) {
 		if (musicType == GUIOMapping[i].type || musicType == (uint32)-1)

common/util.cpp

@@ -381,7 +381,7 @@ String parseGameGUIOptions(const String &str) {
 }
 
 const String getGameGUIOptionsDescription(const String &options) {
-	String res = "";
+	String res;
 
 	for (int i = 0; g_gameOptions[i].desc; i++)
 		if (options.contains(g_gameOptions[i].option[0]))

engines/game.h

@@ -65,12 +65,12 @@ enum GameSupportLevel {
 class GameDescriptor : public Common::StringMap {
 public:
 	GameDescriptor();
-	GameDescriptor(const PlainGameDescriptor &pgd, Common::String guioptions = "");
+	GameDescriptor(const PlainGameDescriptor &pgd, Common::String guioptions = Common::String());
 	GameDescriptor(const Common::String &gameid,
 	              const Common::String &description,
 	              Common::Language language = Common::UNK_LANG,
 				  Common::Platform platform = Common::kPlatformUnknown,
-				  Common::String guioptions = "",
+				  Common::String guioptions = Common::String(),
 				  GameSupportLevel gsl = kStableGame);
 
 	/**

gui/options.cpp

@@ -141,7 +141,7 @@ void OptionsDialog::init() {
 	_oldTheme = g_gui.theme()->getThemeId();
 
 	// Retrieve game GUI options
-	_guioptions = "";
+	_guioptions.clear();
 	if (ConfMan.hasKey("guioptions", _domain)) {
 		_guioptionsString = ConfMan.get("guioptions", _domain);
 		_guioptions = parseGameGUIOptions(_guioptionsString);
@@ -155,7 +155,7 @@ void OptionsDialog::open() {
 	setResult(0);
 
 	// Retrieve game GUI options
-	_guioptions = "";
+	_guioptions.clear();
 	if (ConfMan.hasKey("guioptions", _domain)) {
 		_guioptionsString = ConfMan.get("guioptions", _domain);
 		_guioptions = parseGameGUIOptions(_guioptionsString);

gui/saveload.cpp

@@ -92,7 +92,7 @@ int SaveLoadChooser::runModalWithPluginAndTarget(const EnginePlugin *plugin, con
 	_thumbnailSupport = _metaInfoSupport && (*_plugin)->hasFeature(MetaEngine::kSavesSupportThumbnail);
 	_saveDateSupport = _metaInfoSupport && (*_plugin)->hasFeature(MetaEngine::kSavesSupportCreationDate);
 	_playTimeSupport = _metaInfoSupport && (*_plugin)->hasFeature(MetaEngine::kSavesSupportPlayTime);
-	_resultString = "";
+	_resultString.clear();
 	reflowLayout();
 	updateSaveList();