SI-8117 Fix bug when mixing well-positioned named and positional args

The method `missingParams` which returns undefined parameters of
a given invocation expression still assumed that named arguments can
only appear after positional ones.
Syntax: `namedArgsOCP.forall { arg => val n = etc ; n.isEmpty || etc }`.

The brace means you don't need your other brace, IIRC, where IIRC = I didn't try it.