FieldAccessDescriptorParser failures throw NPE

An actual parser error is logged, but then a null pointer exception is thrown.

Example code:

FieldAccessDescriptor.withFieldNamesAs(ImmutableMap.of("\"", "\""));

Logs:

line 1:0 token recognition error at: '"'
line 1:1 mismatched input '' expecting {IDENTIFIER, '*'}

Imported from Jira [BEAM-13696](https://issues.apache.org/jira/browse/BEAM-13696). Original Jira may contain additional context.
Reported by: ibzib.