The cause of the problem seems to be, that in mod_url_mod_form (/mod/url/mod_form.php line 54) the 'externalurl' field type is PARAM_URL and when inserting URL with space in end, then it will not be valid URL and the value gets empty.

When changing the described line as the following:

-        $mform->setType('externalurl', PARAM_URL);

+        $mform->setType('externalurl', PARAM_TEXT);

then the form starts to work as expected.
The URL gets validated anyway in function "validation" in the same file.

Create/edit URL resource and try to enter URLs that are valid or not valid, with or without extra spaces in the beginning/end.
Try to leave field empty or containing just spaces.

Make sure that empty (or ' ') URL is not accepted, extra spaces are trimmed.

Please note that URL validation is very light here.