BLF: Don't Print Empty Strings

Optimize font drawing by skipping empty strings.

See D15472 for more details.

Differential Revision: https://developer.blender.org/D15472

Reviewed by Campbell Barton

source/blender/blenfont/intern/blf_font.c

@@ -836,7 +836,7 @@ static void blf_font_boundbox_foreach_glyph_ex(FontBLF *font,
   size_t i = 0, i_curr;
   rcti gbox_px;
 
-  if (str_len == 0) {
+  if (str_len == 0 || str[0] == 0) {
     /* early output. */
     return;
   }