Bug 11779: (follow-up) improve GetLoggedInBranchcode() and add test cases

Bug 11779: (follow-up) improve GetLoggedInBranchcode() and add test cases

This patch adjusts the new GetLoggedInBranchcode() template function so
that it returns the empty string rather than undef if there is no
active user environment.  That way, there won't be lots of undefined
value warnings if/when this function gets used in the OPAC.

This patch also adds test cases.

To test:

[1] Verify that there are no regressions in the main test
    plan for this bug.
[2] Verify that prove -v t/db_dependent/Koha_template_plugin_Branches.t
    passes.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 065a3a5efa9b90723460a607ac85591c17945b00)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>