gccrs: fix ICE segfault with empty feature gate

gccrs: fix ICE segfault with empty feature gate

This patch fixes an issue where an empty feature gate would segfault,
instead of reporting a syntax error to the user.

gcc/rust/ChangeLog:

	* ast/rust-ast.h: (AST::Attribute): add empty_input function
	* checks/errors/rust-feature-gate.cc: (FeatureGate::visit): check for empty feature gate

gcc/testsuite/ChangeLog:

	* rust/compile/feature.rs: add an invalid empty feature to produce an error

Signed-off-by: Matty Kuhn <matty.kuhn.1@gmail.com>