Executing match-get with only anonymous variables should be disallowed

[vmax]

Description

match-get with only anonymous variables should be disallowed as executing such a query yields empty concept maps as a result.

Environment

1. OS (where Grakn server runs): macOS Catalina
2. Grakn version (and platform): 1.6.0
3. Grakn client: client-java
4. Other environment details:

Reproducible Steps

Execute match(var().isa(…)).get()

Expected Output

Syntax error

Actual Output

Query executed with empty ConceptMaps as a result