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 <#337>.

app/src/server.c

@@ -93,7 +93,7 @@ static process_t execute_server(const char *serial,
         max_size_string,
         bit_rate_string,
         tunnel_forward ? "true" : "false",
-        crop ? crop : "''",
+        crop ? crop : "-",
         send_frame_meta ? "true" : "false",
     };
     return adb_execute(serial, cmd, sizeof(cmd) / sizeof(cmd[0]));

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

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