Fix android hook overload filter.

[jpacg]

android hooking watch class_method android.util.Base64.decode --dump-args


An unexpected internal exception has occurred. If this looks like a code related error, please file a bug report!
'NoneType' object has no attribute 'replace'

Python stack trace: Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/objection/console/repl.py", line 371, in start_repl
    self.run_command(document)
  File "/usr/local/lib/python3.7/site-packages/objection/console/repl.py", line 185, in run_command
    exec_method(arguments)
  File "/usr/local/lib/python3.7/site-packages/objection/commands/android/hooking.py", line 139, in watch_class_method
    overload_filter.replace(' ', ''),
AttributeError: 'NoneType' object has no attribute 'replace'

[leonjza]

Thank you!