phase1: fix crash on exception during Git update

phase1: fix crash on exception during Git update

Fixes following exception:

 2023-05-15 14:08:56+0000 [-] <Build master_apm821xx/nand number:1 results:success> build got exception when running step Git(name='git', repourl='https://git.openwrt.org/openwrt/openwrt.git', mode='full', method='fresh', locks=renderer(<function NetLockDl at 0x7f5d33351ee0>), haltOnFailure=True)
 2023-05-15 14:08:56+0000 [-] Unhandled Error
	Traceback (most recent call last):
	  File "/usr/local/lib/python3.9/dist-packages/twisted/internet/defer.py", line 1792, in gotResult
	    _inlineCallbacks(r, gen, status, context)
	  File "/usr/local/lib/python3.9/dist-packages/twisted/internet/defer.py", line 1693, in _inlineCallbacks
	    result = context.run(
	  File "/usr/local/lib/python3.9/dist-packages/twisted/python/failure.py", line 518, in throwExceptionIntoGenerator
	    return g.throw(self.type, self.value, self.tb)
	  File "/usr/local/lib/python3.9/dist-packages/buildbot/process/build.py", line 576, in _start_next_step_impl
	    log.err(e)
	--- <exception caught here> ---
	  File "/usr/local/lib/python3.9/dist-packages/buildbot/process/build.py", line 562, in _start_next_step_impl
	    results = yield step.startStep(self.conn)
	builtins.KeyError: None

as None is valid property value, but cant be used as dict index.

Signed-off-by: Petr Štetiar <ynezz@true.cz>