Return no responses in IterResponses if timeline model is None.
Return no responses in IterResponses if timeline model is None.

In some tests we don't get any traffic, which causes an excpetion in
IterResponses when self._timeline_model is None when it's not expected
to be.  In particular this is causing a test to fail when an extra via
header is expected.

BUG=576308

Review URL: https://codereview.chromium.org/1578823002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#368677}

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