input attribute of stardoc rule is not optional, despite what the documentation says

[shs96c]

The documentation for stardoc says that the input attribute is optional. However, leaving out this attribute leads to the following error:

ERROR: /foo/rules_kotlin/kotlin/BUILD:38:8: in stardoc rule //kotlin:stardoc: 
Traceback (most recent call last):
	File "/private/var/tmp/_bazel_shs/71d26a4c57cf01b6c7f0c561a4a031b5/external/io_bazel_stardoc/stardoc/stardoc.bzl", line 29, column 25, in _stardoc_impl
		input_files = depset(direct = [ctx.file.input], transitive = [
Error in depset: cannot add an item of type 'File' to a depset of 'NoneType'

Either the field is mandatory, or a check must be done to correctly identify whether input has been set.