common: Fix whitespace CTCP ignore invalid index

When constructing a CTCP ignore, check that splitting on whitespace
results in non-empty list before taking from the list.  If it's
empty, fall back to assuming any valid sender, i.e. "*"/".*"
depending on whether the rule is set as wildcard or regex.

This fixes issues when receiving a CTCP with an invalid CTCP ignore
rule such as "  ".