Bug 15494: (QA follow-up) Additional polishing

Bug 15494: (QA follow-up) Additional polishing

[1] Fix two typos in Circulation.t.
    Although the test does not fail, line 2127 contains two typos.
    Changing INVISILE to INVISIBLE :)
    And type should be itype.
[2] Remove $yaml as leftover from older code.
[3] Add a next when the split on /:/ does not give two results. This will
    prevent uninit warnings (although still disabled now in Circulation).
[4] For the same reason we should switch the lines for NULL and empty
    string. The undefs you insert should trigger a warn.
[5] The line for empty string should not insert undef, but empty string.
    For the same reason adding the condition defined($_) ...
    And proving it by adding two tests for the opposite values of
    callnumber and itemnotes.
[6] Adding a strip spaces around the fieldname. User friendly..

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>