common/collections: Fix append regression to allow appending nil

Closes #11180

common/collections/append_test.go

@@ -74,6 +74,9 @@ func TestAppend(t *testing.T) {
 			[]any{"c"},
 			false,
 		},
+		{[]string{"a", "b"}, []any{nil}, []any{"a", "b", nil}},
+		{[]string{"a", "b"}, []any{nil, "d", nil}, []any{"a", "b", nil, "d", nil}},
+		{[]any{"a", nil, "c"}, []any{"d", nil, "f"}, []any{"a", nil, "c", "d", nil, "f"}},
 	} {
 
 		result, err := Append(test.start, test.addend...)