Bug 19088: plugins-upload causes error log noise

Bug 19088: plugins-upload causes error log noise

After uploading a plugin the error log indicates
use of uninitialized value in $op. This patch
silences the noise.

To test:
1) Set <enable_plugins> to 1 (one) in koha-conf.xml.
2) Set the UseKohaPlugins system preference to 'Enable'.
3) Navigate to Administration -> Manage plugins.
4) Install the test plugin KPZ file attached to this bug.
5) Notice the uninitialized value noise in the error log.
6) Uninstall the plugin (plack restart may be required if plack is
    enabled).
7) Apply patch.
8) Install the plugin again.
9) Notice no noise in the error log.
10) Run qa tools.
11) Run prove t/db_dependent/Plugins.t

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>