Add null check around mImeRequester.getWindow()

mImeRequester is an InsetsControlTarget, which in some cases isn't
backed by a WindowState. Therefore we can't write window dump to
protobuf without checking if mImeRequester.getWindow() is null first.

Bug: 186006740
Test: Builds.
Change-Id: I3648763b4cee3a3d5cd1d39ebacbb00289f6fb33