BUG/MINOR: stick-tables: Missing stick-table key nullity check

BUG/MINOR: stick-tables: Missing stick-table key nullity check

This bug arrived with this commit:
     MAJOR: stktable: split the keys across multiple shards to reduce contention

At this time, there are no callers which call stktable_get_entry() without checking
the nullity of <key> passed as parameter. But the documentation of this function
says it supports this case where the <key> passed as parameter could be null.

Move the nullity test on <key> at first statement of this function.

Thanks to @chipitsine for having reported this issue in GH #2518.