Allow null pointer when changing font size

[Jack-Ji]

According to docs from imgui.h:

 // Parameters stacks (font) // - PushFont(font, 0.0f) // Change font and keep current size // - PushFont(NULL, 20.0f) // Keep font and change current size // - PushFont(font, 20.0f) // Change font and set size to 20.0f // - PushFont(font, style.FontSizeBase * 2.0f) // Change font and set size to be twice bigger than current size. // - PushFont(font, font->LegacySize) // Change font and set size to size passed to AddFontXXX() function. Same as pre-1.92 behavior.