fix: allow `requires`, `ensures` etc to be empty, as in `verus_syn`

[mkovaxx]

Currently verus-analyzer expects at least one Expr in each clause:

verus-analyzer/crates/syntax/rust.ungram

Lines 760 to 767 in 9f98767

	RequiresClause =
	'requires' (Expr (',' Expr)* ','?)
	EnsuresClause =
	'ensures' (Expr (',' Expr)* ','?)
	DefaultEnsuresClause =
	'default_ensures' (Expr (',' Expr)* ','?)

However, verus_syn accepts empty clauses, and that's confirmed to be intentional.

The goal here is to bring verus-analyzer's grammar in sync with verus_syn's.

See discussion here:
verus-lang/verus#2098 (comment)

[Marsman1996]

Will this fix the #70 ?

[mkovaxx]

@Marsman1996 It might, but I'd like to second @parno's suggestion: to parse Verus code reliably, depend on verus_syn or tree-sitter-verus instead of verus-analyzer.