fix issue with block_query_wait setting
fix issue with block_query_wait setting

The block_query_wait field is a pointer type on the struct. Nil typed
fields always need to be checked vs. nil in Merge() functions. I think
this was missed as it was put below Once which is of type bool and
doesn't need it (ie. followed bad pattern).

hashicorp/consul-template,hashicorp/consul-template