[Bug #20322] Fix rb_enc_interned_str_cstr null encoding

	The documentation for `rb_enc_interned_str_cstr` notes that `enc` can be
	a null pointer, but this currently causes a segmentation fault when
	trying to autoload the encoding. This commit fixes the issue by checking
	for NULL before calling `rb_enc_autoload`.

[Bug #20421]

The bug was fixed in Ruby 3.3 via 9dcdffb8bf8a3654fd78bf1a58b30c8e13888a7a

	Added assertion values for Amazon Linux 2023

	---
	 spec/ruby/core/file/utime_spec.rb | 8 +++++---
	 1 file changed, 5 insertions(+), 3 deletions(-)

	Fix support for dynamic keys. (#8273)

	* Skip RBS test.
	---
	 cont.c                               |  8 +++-----
	 spec/ruby/core/fiber/storage_spec.rb | 14 ++++++++++++++
	 tool/rbs_skip_tests                  |  2 ++
	 3 files changed, 19 insertions(+), 5 deletions(-)