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`.

rb_fstring is a private API, so we should use rb_str_to_interned_str
instead, which is a public API.

In the extension libraries in spec/ruby/optional/capi, do not care
about deprecated declarations.