fix(input): ignore min/max if they are empty on all input types

[gkalpak]

When the min/max attributes are empty (i.e. attrs.min/max === ''), there should be no min/max validation applied (i.e. all values should be valid wrt min/max). This works correctly for input[number], but not for date input family (input[date/datetime-local/time/week/month]).

In the case on input[number], an empty string for attrs.min/max is translated to undefined for minVal/maxVal and a check for isUndefined(minVal/maxVal) ensures that no min/max validation takes place.
For the data input family, an empty string for attrs.min/max is translated to NaN (by parseDate()), so an additional check (for isNaN(minVal/maxVal)) is required.

Fixes #12363

[Narretz]

Couldn't we check the emptiness inside parseObservedDateValue and return undefined in that case, too? It'd be one check less for the actual validator to make.

[gkalpak]

Good point, @Narretz. Updated.

[Narretz]

LGTM after squashing the commits!

[gkalpak]

Thx @Narretz ! _{(I just added the extra commits for "reviewability".)}

Squashing and merging...