fix(parser): make empty catch an incomplete.

[ckipp01]

When parsing a TRY if there is an empty catch block instead of just
returning a syntax error return an incomplete if you're at the EOF. This
ensures that in the REPL if you are in a position like:

scala> try {
     | ???
     | } catch

And you hit enter you'll still be able to continue.

Fixes #4393

[som-snytt]

Thanks, I was able to close my ticket as a duplicate, as my ticket number is 10,000 higher.

[odersky]

LGTM