Always use non-empty arguments

The client passes parameters to the server via "adb shell" arguments.

Use "-" instead of "" when no crop is specified to avoid empty
arguments, which are not handled the same way on all devices.

Fixed <https://github.com/Genymobile/scrcpy/issues/337>.

server/src/main/java/com/genymobile/scrcpy/Server.java

@@ -71,7 +71,7 @@ public final class Server {
     }
 
     private static Rect parseCrop(String crop) {
-        if (crop.isEmpty()) {
+        if ("-".equals(crop)) {
             return null;
         }
         // input format: "width:height:x:y"