search-paths: Allow specs with #f as their separator.

search-paths: Allow specs with #f as their separator.

This adds support for single-entry search paths.
Fixes <http://bugs.gnu.org/25422>.
Reported by Leo Famulari <leo@famulari.name>.

* guix/search-paths.scm (<search-path-specification>)[separator]:
Document as string or #f.
(evaluate-search-paths): Add case for SEPARATOR as #f.
(environment-variable-definition): Handle SEPARATOR being #f.
* guix/build/utils.scm (list->search-path-as-string): Add case for
SEPARATOR as #f.
(search-path-as-string->list): Likewise.
* guix/build/profiles.scm (abstract-profile): Likewise.
* tests/search-paths.scm: New file.
* Makefile.am (SCM_TESTS): Add it.
* tests/packages.scm ("--search-paths with single-item search path"):
New test.
* gnu/packages/version-control.scm (git)[native-search-paths](separator):
New field.