When moving strip, or it's handle, text with frame number near handle
should be drawn. This feature was broken by 8d53ead69b, because
function `UI_view2d_text_cache_add` sets all fields of `v2s->rect` to 0.
This case was checked in function `UI_view2d_text_cache_draw`, but it
was not quite obvious.
This commit reverts 8d53ead69b, makes condition for 0 size rectangle
more obvious and adds comment for clarity.
function `UI_view2d_text_cache_add` is only used in NLA and VSE, and
this new condition fits existing use-cases. Status of T97500 is not
affected by this change.
f404dd0b3c