patch 9.0.1727: minor problems with the teapot

Problem: minor problems with the teapot()
Solution: remove the null check, update documentation

Update the builtin-function-list entry. (It implicitly returns 0, but such
functions usually use "none")

Also, tv_get_string_strict() can not return NULL, so remove that check

closes: #12647

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Sean Dewar <seandewar@users.noreply.github.com>

runtime/doc/builtin.txt

@@ -156,7 +156,7 @@ digraph_setlist({digraphlist})	Boolean	register multiple |digraph|s
 echoraw({expr})			none	output {expr} as-is
 empty({expr})			Number	|TRUE| if {expr} is empty
 environ()			Dict	return environment variables
-err_teapot()			Number	produce error 418
+err_teapot([{expr}])		none	give E418, or E503 if {expr} is |TRUE|
 escape({string}, {chars})	String	escape {chars} in {string} with '\'
 eval({string})			any	evaluate {string} into its value
 eventhandler()			Number	|TRUE| if inside an event handler