Don't count zero width space in hasGlyph layout

For sequences that may form ligatures (ie not just a a base
character plus variation selector pair), hasGlyph requests a layout
and then counts the glyph. In some cases, when the source string
contains a variation selector, the layout contains space glyphs with
no associated width, which causes a "false" result. This patch
filters out such zero width spaces.

Bug: 30056285
Change-Id: I9b5db4aa5ad24bf43e237c98b610fd269f2b91c9