To Set it:
import com.atlassian.jira.component.ComponentAccessor
def customFieldManager = ComponentAccessor.customFieldManager
def crtField = customFieldManager.getCustomFieldObjects(issue).find{it.name == "Customer Request Type"}
issue.setCustomFieldValue(crtField, "Desired Request Type")
To Clear it:
This works better than using JSU to "Clear Field Value"; as JSU does not clear it.
import com.atlassian.jira.component.ComponentAccessor
def customFieldManager = ComponentAccessor.customFieldManager
def crtField = customFieldManager.getCustomFieldObjects(issue).find{it.name == "Customer Request Type"}
issue.setCustomFieldValue(crtField, null)
After the script, You can use "IS EmPTY" on the field.