Fix NoneType exception for interface dependencies.

[UebelAndre]

Before this change any dependency with CcInfo which did not contain static or dynamic libraries (e.g. an interface library which has it's own fields for storing the underlying files) would result in the following error:

ERROR: /Users/user/Code/rules_rust/test/unit/cc_info/BUILD.bazel:4:19: in rust_shared_library rule //test/unit/cc_info:rust_dylib_with_interface_lib_dep:
Traceback (most recent call last):
	File "/Users/user/Code/rules_rust/rust/private/rust.bzl", line 126, column 32, in _rust_shared_library_impl
		return _rust_library_common(ctx, "cdylib")
	File "/Users/user/Code/rules_rust/rust/private/rust.bzl", line 190, column 32, in _rust_library_common
		return rustc_compile_action(
	File "/Users/user/Code/rules_rust/rust/private/rustc.bzl", line 1541, column 41, in rustc_compile_action
		dynamic_libraries = ctx.runfiles(files = [
Error in runfiles: at index 0 of files, got element of type NoneType, want File

This pull request fixes this exception.

[illicitonion]

Thanks!