Take a look at building a bool query with two should clauses. You can use a bool query as one of the should clauses in order to invert an exists query using a must_not clause
{
"query": {
"bool" : {
"should" : [
{ "bool" : {
"must_not" : [
{ "exists": { "field" : "brand_name" } }
]
}
},
{ "term" : { "brand_name" : "some brand name" } }
]
}
}
}