Optimize font drawing by skipping empty strings.

blf_font_draw_ex contains an early-out test that the passed str_len is not zero. Unfortunately str_len is the byte size of the string buffer, not the string length. We never send a zero-length buffer to the function but we do send empty strings to it all the time. I count at least 60 during startup alone.

This patch just adds a test for empty string to that early exit, which basically just avoids unnecessary calls to blf_batch_draw_begin and blf_batch_draw_end