Search for empty values should be allowed

[amir-zeldes]

Sometimes corpora contain empty string values, such as tokens with no characters (for traces etc.). For example in OntoNotes, there are many empty tokens that should be findable using:

tok=""

Right now the parser refuses to execute this query, saying:

"Semantic error: The validated object is null"

The desired behavior is finding all values (of span or anno_value) where the string value is the empty string.