KRACOEUS-8345 added getParentBudget to allow searching

Until PD's can create valid IPs that can fund an award, this isn't fully testable. However, this prevents a stack trace from popping up if you click the search icon next to an award budget period line.
Without this change, when clicking on the period search link on an award budget, the following error happens before the search screen displays:
java.lang.RuntimeException: error getting property value for class org.kuali.kra.award.budget.AwardBudgetForm.document.parentDocument.award.awardNumber Unknown property 'parentDocument' on class 'class org.kuali.kra.award.budget.document.AwardBudgetDocument'
at org.kuali.rice.krad.util.ObjectUtils.getPropertyValue(ObjectUtils.java:339)

(See the associated jira for the full trace)

How would you prefer this get solved?  I didn't want to potentially break proposal budget, so this seemed like the least impactful solution.