FrameCapture: Handle null texture in MEC When interating through a UnsafeResourceMapIter, we need to check for null entries before dereferencing. Test: Fortnite MEC Bug: b/180418810 Change-Id: I0dc234234f9bb219026b2b67febb32e8af19e8b1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6839286 Reviewed-by: Xiang Wang <xwxw@google.com> Auto-Submit: Cody Northrop <cnorthrop@google.com> Commit-Queue: Xiang Wang <xwxw@google.com>