"allowEmpty" property for UCI options. Disallow empty values for net file.

[Sopel97]

This seems to break the case when EvalFile is set to blank:

setoption name EvalFile value
go infinite
info string ERROR: If the UCI option "Use NNUE" is set to true, network evaluation parameters compatible with the engine must be available.
info string ERROR: The option is set to true, but the network file  was not loaded successfully.
...

It's useful to set this to blank in a GUI (e.g. cutechess GUI) so that when you download new dev versions with new nets, you don't have to manually change the EvalFile parameter each time. I thought this was supported functionality (but I could be wrong)

This PR brings back the previous behaviour for this particular uci option, as setting empty strings as paths to the networks doesn't make much sense we should revert to the previous.

[vondele]

closing in favor of #3678