displayio: ColorConverter: fix logic errors about transparent pixels

The transparent_color field was never initialized.  I _think_ this means
its value was always set to 0, or the blackest of blacks.  Instead,
initialize it to the sentinel value, newly given the name
NO_TRANSPARENT_COLOR.

This exposed a second problem: The test for whether there was an existing
transparent color was wrong (backwards).  I am guessing that this was not
found due to the first bug; since the converter had a transparent color,
the correct test would have led to always getting the error "Only one
color can be transparent at a time".

Closes #3723

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