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.