In fonts with only upper-case glyphs, that are also missing a space glyph, `get spaceWidth` won't be able to return anything useful.
By adding upper-case `I` as a fallback, we can thus improve text-selection in some PDF files.
Note that locally, the patch causes slight movement in a few existing `text` tests, but in my opinion this actually looks like slight improvements.
Fixes 7180.
