detect/dataset: fix space condition in rule lang

detect/dataset: fix space condition in rule lang

If there is a space following a keyword that does not expect a value,
the rule fails to load due to improper value evaluation.
e.g. Space after "set" command
alert http any any -> any any (http.user_agent; dataset:set  ,ua-seen,type string,save datasets.csv; sid:1;)

gives error
[ERRCODE: SC_ERR_UNKNOWN_VALUE(129)] - dataset action "" is not supported.

Fix this by handling values correctly for such cases.

(cherry picked from commit 6d2a2a073120906304f70c0e120565eae96e36b8)