GetCapture can reuse the bool pointer argument of GenericCaptureGetter
instead of duplicating that logic with additional checks.

The check also incorrectly checks for undefined while
GenericCaptureGetter returns the empty string on failure.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2422563002
Cr-Commit-Position: refs/heads/master@{#40309}