[CTFE] Skip empty custom webpages instead of reporting error
[CTFE] Skip empty custom webpages instead of reporting error

This is to allow the following input in the UI which nednguyen@ tried yesterday:
"""
webpage1,
webpage2,
"""

BUG=skia:5886

Change-Id: I384f07a6705b493aada26d1e3be201bc1b4d277b
Reviewed-on: https://skia-review.googlesource.com/4586
Reviewed-by: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot