make func TestSearch.unknown_word check for nil

[1Mark]

otherwise the refactor contains a regression

[1Mark]

Alternatively, this logic could be added to the assertErrors function
I.e it could be like

func assertError(t testing.TB, got, want error) {
	t.Helper()
	if want != nil && got == nil {
		t.Fatal("expected to get an error")
	}
	if got != want {
		t.Errorf("got error %q want %q", got, want)
	}
}

However, it will mean that a rewrite will be required for other sections of maps.md (such as here https://quii.gitbook.io/learn-go-with-tests/go-fundamentals/maps#write-the-minimal-amount-of-code-for-the-test-to-run-and-check-the-output-2)

(adding screenshot since I can't get a permalink from the site).