Reject braced constructor field without value

[makenowjust]

Fixes #418.

parseBracedNewConstructorField switched on : and { but had no default case, so a field followed by anything else (e.g. NEW A00{A0}, where the field is followed by }) produced a BracedConstructorField with a nil Value — and err == nil. SQL() and the position accessors then dereferenced the nil value and panicked.

A braced constructor field requires a value, so the parser now reports it as a syntax error:

syntax error: :1:15: expected token: {, :, but: }
  1|  NEW foo { bar }
   |                ^

Error recovery wraps the expression in an ast.BadExpr as usual.

Tests

• New testdata case testdata/input/expr/!bad_new_braced_constructor_field_without_value.sql (NEW foo { bar }) with its generated result, alongside the existing !bad_new_braced_constructor.sql.

[makenowjust]

@apstndb Could you review this?

[apstndb]

LGTM

[makenowjust]

This error looks strange. We want to avoid it.

[makenowjust]

Furthermore, this lacks the closing brace }. It seems actually a bug.