NPE during simple filter parsing

[ribrewguy]

I haven't dug into the issue, but when parsing a simple filter principal == 035489f4-d602-46c5-b568-39d1f852b565 I get an NPE:

Caused by: java.lang.NullPointerException
	at com.github.vineey.rql.querydsl.filter.QuerydslRsqlVisitor.visit(QuerydslRsqlVisitor.java:80) ~[rsql-querydsl-filter-2.0.0.RELEASE.jar:?]
	at com.github.vineey.rql.querydsl.filter.QuerydslRsqlVisitor.visit(QuerydslRsqlVisitor.java:37) ~[rsql-querydsl-filter-2.0.0.RELEASE.jar:?]
	at cz.jirutka.rsql.parser.ast.ComparisonNode.accept(ComparisonNode.java:70) ~[rsql-parser-2.1.0.jar:2.1.0]
	at com.github.vineey.rql.querydsl.filter.QuerydslFilterBuilder.visit(QuerydslFilterBuilder.java:34) ~[rsql-querydsl-filter-2.0.0.RELEASE.jar:?]
	at com.github.vineey.rql.querydsl.filter.QuerydslFilterBuilder.visit(QuerydslFilterBuilder.java:31) ~[rsql-querydsl-filter-2.0.0.RELEASE.jar:?]
	at com.github.vineey.rql.filter.parser.DefaultFilterParser.parse(DefaultFilterParser.java:42) ~[rsql-api-filter-2.0.0.RELEASE.jar:?]