REF: implement maybe_promote_scalar
Narrowing down the set of cases where we have to worry about a non-scalar fill_value.  I'm pretty sure we can narrow it down further, but saving that for a separate step as it has some complications for EA corner cases.

cc @jreback
lgtm. ping on green. ping gentle ping.  we're getting close to being done with the maybe_promote saga Unnecessary following #29416, closing.