"is-date" should return "true" only if it's a date with nothing in "parts"

[erazlogo]

from an im with dan s.:
"is-date" should return "true" if it's a date with nothing in "parts", and fallback to the whole string otherwise.

right now "is-date" seems to return "true" if the field can be parsed into a valid date (regardless of what else it contains), which leads to the following loss of data when moving from the Zotero date field to CSL:

"1750-1754" > "1750"
"[2000?]" > "2000"
"ca. 2000" > "ca 2000"
"n.d." > "n.d"

All of the above should probably evaluate as "false" and just return the whole string.