fix: make categories under Feed an optional type

[totto2727]

Feed["categories"] is defined as Category[].
However, it is actually Category[] | undefined.

The process is to assign a value to categories after it is cast to Feed. However, just before that, it is verified with if, and the assignment may be skipped. This validation also indicates to the compiler that it is NonNullable and the assignment to Category[] is allowed in the if statement.

code in the relevant section

rss/src/mappers/mapper.ts

Lines 379 to 384 in ebee31f

	if (categories) {
	result.categories = categories?.map((category) => ({
	term: category.term,
	label: category.label,
	}));
	}

The fundamental solution is to refactor the process with type assertions, but since this would be a major change, only the type definitions are modified in this PR.

The main changes are the following commits

6d98b1f