avoid committing rows just to figure out rendition, blink and cursor (#19984)

We have been looking at a set of conhost crashes due to low-memory
conditions. I've observed that they all happen during the first pass of
rendering, before there's even an engine set up, when we try to estimate
whether there are any blink attributes. I also found that we'll commit
the buffer to check the cursor's double-width status and whether it's on
a double-width-rendition line.

Uncommitted rows _never_ contain a blinker, are never double-width and
never contain DBCS.

We can't necessarily avoid committing an empty buffer _forever,_ but
this at least moves the first commit until after the renderer truly
starts.

This prevents us from committing an empty buffer for headless console
apps (!)