font_fallback, use language prio only if id prio not set

Corrected check for "not priority" to "priority is None"
in order to also allow value set to 0.
Previously 0 and None value would have been treated the same.

(cherry picked from https://partner-android-review.googlesource.com/q/commit:709479ff0383007a8e11b6a97dcc83cb042639b4)

Test: atest frameworks/base/data/fonts/script/test
Bug: 414480576
Flag: NONE build only fix
Change-Id: I635f3eef37c83195ad5d901fefb2bcba0f2a2110