Description of problem:
Trying to set an empty custom property results in an error on the engine: 
2013-12-26 09:55:46,700 WARN  [org.ovirt.engine.core.bll.network.vm.UpdateVnicProfileCommand] (http-/0.0.0.0:8080-1) [246beb72] CanDoAction of action UpdateVnicProfile failed. Reasons:VAR__TYPE__VNIC_PROFILE,VAR__ACTION__UPDATE,ACTION_TYPE_FAILED_INVALID_CUSTOM_PROPERTIES_INVALID_SYNTAX



Version-Release number of selected component (if applicable):


How reproducible:
Always


Steps to Reproduce:
1. Create a custom property allowing an empty value, such as:
engine-config -s CustomDeviceProperties="{type=interface;prop={someProp=([a-z0-9A-Z]*)$}}" --cver=3.3
2. Add a network on a 3.3 data center.
3. Add a network profile and on the dialog select your "someProp" property but don't enter any value.
4. Click OK to add the profile.

Actual results:
You will get an error "Error while executing action: Cannot edit VM network interface profile if custom properties are in invalid format. Please check the input."


Expected results:
The profile should be saved without error.


Additional info:
The error comes from a check in org.ovirt.engine.core.utils.customprop.DevicePropertiesUtils.validateProperties(Version, VmDeviceGeneralType, Map<String, String>)

This method invokes syntaxErrorInProperties(properties) which is meant for checking the property definition in the configuration, not the actual uasge of the properties and their conformity to the syntax (this check is done later in the validateProperties method).

This should probably be fixed also for other usages of custom properties.