RubyLB: Fix a crash in ShapeResult::ApplyLeadingExpansion()

We can't add spaces to a ShapeResult with no glyphs.  So
ExpandableItemsFinder should not assume such ShapeResults expandable.

Bug: 345308888
Change-Id: I8998d7119bad3a506b2e4a0b2238a6e15995e425
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5611320


Reviewed-by: Koji Ishii <kojii@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312599}