Allow trailing commas in short-lamba expressions

Summary: As it says on the tin.

The parser allows it already, but only if the scanner has been
type for '(' and ')' have been correctly overridden to be
T_LAMBDA_OP and T_LAMBDA_CP.

tryParseNonEmptyLambdaParams() was incorrectly disallowing
trailing commas.

{sync, type="child", parent="internal", parentrevid="1748938", parentrevfbid="735574109854167", parentdiffid="5993857"}

Reviewed By: @paulbiss

Differential Revision: D1748938

Signature: t1:1748938:1418936065:e0c4d563bbbeddc1ff8d20f934685089242235d3