Driver doesn't support app to be none

[netd-eric]

I need to launch already installed app, but if I set the app capability to None it will raise error:

Traceback (most recent call last):
File "/Users/ericcheung/company_prj/talentslate_appiumtests/TalentSlate.AppiumTests/tests/native/test_android.py", line 67, in setUp
'automationName': 'selendroid',
File "/Users/ericcheung/.virtualenvs/talentslate_appiumtests/lib/python2.7/site-packages/appium/webdriver/webdriver.py", line 36, in init
super(WebDriver, self).init(command_executor, desired_capabilities, browser_profile, proxy, keep_alive)
File "/Users/ericcheung/.virtualenvs/talentslate_appiumtests/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 98, in init
self.start_session(desired_capabilities, browser_profile)
File "/Users/ericcheung/.virtualenvs/talentslate_appiumtests/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 188, in start_session
response = self.execute(Command.NEW_SESSION, parameters)
File "/Users/ericcheung/.virtualenvs/talentslate_appiumtests/lib/python2.7/site-packages/selenium/webdriver/remote/webdriver.py", line 252, in execute
self.error_handler.check_response(response)
File "/Users/ericcheung/.virtualenvs/talentslate_appiumtests/lib/python2.7/site-packages/selenium/webdriver/remote/errorhandler.py", line 194, in check_response
raise exception_class(message, screen, stacktrace)
WebDriverException: Message: An unknown server-side error occurred while processing the command. Original error: Could not find app apk at 'undefined