WebView: guard against null TestProcessClient

It's possible for createProcessB to throw an exception if we couldn't
successfully create the process. In this case, the @After tearDown step
still runs, so we need to guard against this case or else we'll emit a
second crash stack.

Fixes: 395726149
Test: temporarily patch in a RuntimeException into the setup method
Test: atest CtsWebkitTestCases:PacProcessorTest
Test: Verify the only crash is the one in setup
Flag: TEST_ONLY
Change-Id: Icd5a394e7e41fec1d89cd5587bace99fa666bed5