Some users of minui don't need to draw texts; and we should let them
still be able to use the library without providing a font file. Also add
check for null pointers in gr_measure() and gr_font_size().
Bug: 80535212
Test: boot into recovery without a font file, and the buttons still work
Change-Id: I848e4410f2ce09ea0ab433573e6827b7e9b2c575