Remove log verbosity of UIDReference.get when value is None or empty

[xispa]

Description of the issue/feature this PR addresses

UIDReferenceField.get tries to fetch the object also if the value stored in the field is empty or None. This causes a lot of verbosity in the logs as if were errors, but are not, e.g.:

2017-11-20 16:43:03 ERROR Bika <Analysis at Zn>.Instrument: Resolving UIDReference failed for .  No object will be returned.
2017-11-20 16:43:03 ERROR Bika <Analysis at Zn>.Method: Resolving UIDReference failed for .  No object will be returned.
2017-11-20 16:43:03 ERROR Bika <Analysis at Zn>.Method: Resolving UIDReference failed for .  No object will be returned.
2017-11-20 16:43:03 ERROR Bika <Analysis at Zn>.Method: Resolving UIDReference failed for .  No object will be returned.
2017-11-20 16:43:03 ERROR Bika <Analysis at Zn>.Instrument: Resolving UIDReference failed for .  No object will be returned.

Current behavior before PR

A lot of false errors appear in the log when calling a UIDReferenceField's get function if the field has no value (or is empty if multivalued) .

Desired behavior after PR is merged

No false-errors in the log.

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.