The internal data structures are made of two dicts: .values for values and
.tales for TALES. Formulator expect that these two dicts have keys for all
properties, because of the implementation of Field.has_value which checks
for the presence of the key in .values

When making a field with only TALES, this method was setting the value only
in .tales but the same key needs to be present also in .values, otherwise
get_value would return False for this key.