Avoid copying an empty custom property map

Copying an empty InheritedVariables map during style
resolution causes a Oilpan allocated object to be
created and then over-written immediately with a null
ptr. Avoid this by checking that content is actually
being copied before creating anything.

Bug: 1479492
Change-Id: I59a27d8b265b50bd6dcf75d8d54031d094377b11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4853938
Auto-Submit: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1194730}