Fixes subtitle mapping for video with only external subtitles

src/main/java/net/knarcraft/ffmpegconverter/converter/MkvH264Converter.java

@@ -56,7 +56,7 @@ public class MkvH264Converter extends AbstractConverter {
         }
 
         // Map subtitles if present
-        if (!filterStreamsByType(streams, SubtitleStream.class).isEmpty()) {
+        if (hasInternalStreams(streams)) {
             command.add("-map");
             command.add("0:s");
             command.add("-c:s");
@@ -72,4 +72,20 @@ public class MkvH264Converter extends AbstractConverter {
         return command.toArray(new String[0]);
     }
 
+    /**
+     * Checks whether the processed file has any internal subtitle streams
+     *
+     * @param streams <p>All parsed streams for the video file</p>
+     * @return <p>True if the file has at least one internal subtitle stream</p>
+     */
+    private boolean hasInternalStreams(List<StreamObject> streams) {
+        for (StreamObject subtitleStream : filterStreamsByType(streams, SubtitleStream.class)) {
+            if (((SubtitleStream) subtitleStream).isInternalSubtitle()) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
 }