Issue with merging FacetResults when the first operand contain a FacetResults with nil Terms

Criminal: func (fr *FacetResult) Merge(other *FacetResult){}

Merge method isn't handling the case when fr.Terms == nil. 

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Q) But how did fr.Terms became nil ?? ( It is a struct and as per the usual code flow it is unlikely to have nil value )

Ans)

FacetResult construct implements custom json marshal and unmarshal logic 
( This was introduced in https://github.com/blevesearch/bleve/pull/1404 )

func (tf *TermFacets) MarshalJSON() ([]byte, error) {

    return util.MarshalJSON(tf.termFacets)

}

In case when tf.termFacets is nil, tf will be marshalled as "null" in json object, Thus making fr.Terms nil.