LP1789982: Allow Null Values in Numeric ACP Fields

LP1789982: Allow Null Values in Numeric ACP Fields

Because of the way Angular treats model values there needs
to be a translation between strings and numbers (hence the
str-to-* and *-to-str directives) but this can end up trying
to save empty numeric values as the literal string 'null',
which blows up when saving the item. In order to allow
saving items with empty prices, costs, or deposits, we're
better off treating them as plain strings with a regex
validator for now.

Signed-off-by: Jason Boyer <jboyer@library.in.gov>