Optimize matching of castItems to particDesc

[cmil]

Find ways of improving the matching of castItems to construct the particDesc. One instance that need to be fixed is the usage of "empty-string" in tc2dracor.xq:

https://github.com/dracor-org/fredracor/blob/989f68ef457fc975b95a0456767433e94fb7af38/tc2dracor.xq#L97

This is a follow up issue to #13.

[lucagiovannini7]

Related issue: failed matching in the transformation might also result in the character's sexattribute being defaulted to UNKNOWN.

Here an example from Dalibray's Soliman (fre000425):

• Original XML (sex: 1 = male) <castItem><role id="GENTILHOMME DE SOLIMAN" sex="1" type="H" statut='aristocrate' age='A' stat_amour='néant'>GENTILHOMME DE SOLIMAN</role>.</castItem>

• DraCor XML <person xml:id="un-gentilhomme" sex="UNKNOWN"><!--WARNING: no castItem found for reference in @who--> <persName>Un gentilhomme</persName> </person>

If it is not going to break the workflow, I can correct these mistakes in the plays I'm using.