Avoid triggering Qt warnings about empty file names

Sometimes it may happen that we try to load an image even though the
file name is empty. Just return early from these functions in that case.

src/libtiled/imagecache.cpp

@@ -103,6 +103,9 @@ QHash<TilesheetParameters, CutTiles> ImageCache::sCutTiles;
 
 LoadedImage ImageCache::loadImage(const QString &fileName)
 {
+    if (fileName.isEmpty())
+        return {};
+
     auto it = sLoadedImages.find(fileName);
 
     QFileInfo info(fileName);
@@ -127,6 +130,9 @@ LoadedImage ImageCache::loadImage(const QString &fileName)
 
 QPixmap ImageCache::loadPixmap(const QString &fileName)
 {
+    if (fileName.isEmpty())
+        return {};
+
     auto it = sLoadedPixmaps.find(fileName);
 
     bool found = it != sLoadedPixmaps.end();
@@ -171,6 +177,9 @@ static CutTiles cutTilesImpl(const TilesheetParameters &p)
 
 QVector<QPixmap> ImageCache::cutTiles(const TilesheetParameters &parameters)
 {
+    if (parameters.fileName.isEmpty())
+        return {};
+
     auto it = sCutTiles.find(parameters);
 
     bool found = it != sCutTiles.end();
@@ -199,6 +208,9 @@ void ImageCache::remove(const QString &fileName)
 
 QImage ImageCache::renderMap(const QString &fileName)
 {
+    if (fileName.isEmpty())
+        return {};
+
     static QSet<QString> loadingMaps;
 
     if (loadingMaps.contains(fileName)) {