SVG Text NG: Fix a crash in NGSvgTextLayoutAlgorithm::ResolveTextLength()

This CL fixes a crash in it with an empty textLength element.
NGSvgTextLayoutAttributeBuilder don't need to create an entry for empty
textLength elements.

Bug: 1246373
Change-Id: If5830aa59795553100f40d2a5b743a7d774737f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3140802


Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#918497}