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>.

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]));