PR gh-358 introduced a "scheme but no host" check in the fromHttpUrl()
method in UriComponentsBuilder, but a similar check was not added to
fromUriString() at that time.
This commit introduces a "scheme but no host" check in fromUriString()
to align with the functionality in fromHttpUrl().
Note, however that the regular expressions used to match against the
hostname or IP address are inexact and still permit invalid host names
or IP addresses. True validation of the host portion of the URI is out
of scope for this commit.
Closes gh-25334
a92441186c