Allow optional string values to be cleared in the Physical Library catalogue

At the moment it's not possible to clear optional string values in the Physical Library.

[root@ctacli /]# cta-admin pl ch --name NAME -g URL_VAL
[root@ctacli /]# cta-admin pl ch --name NAME -g ""
bindString failed: paramName=:GUI_URL: bindString failed for SQL statement UPDATE PHYSICAL_LIBRARY SET GUI_URL = :GUI_URL,LAST_UPDATE_USER_NAME = :LAST_...: Optional string parameter :GUI_URL is an empty string. An optional string parameter should either have a non-empty string value or no value at all.

This is a bug. Nullable string values should be clearable.