No validation for correct ClassNameId in FragmentEntryLink, breaking FragmentEntryLink viewUsages if classNameId is invalid

• Type: Bug
• Status: Closed
• Resolution: Fixed
• Affects Version/s: Master
• Fix Version/s: 7.2.10 DXP FP1, 7.2.X, 7.2.1 CE GA2, Master
• Component/s: Fragment Administration
• Labels:
  • liferay-fixpack-dxp-1-7210
• Branch Version/s:
  7.2.x
• Backported to Branch:
  Committed
• Fix Priority: 3
• Git Pull Request: https://github.com/brianchandotcom/liferay-portal/pull/76128

Description
Many times we assume the classNameId / classPK pair is correct when creating FragmentEntryLinks. It seems like better practice to validate that the classNameId / classPK pair is valid upon creation to prevent unexpected errors further down the road.

Steps to Reproduce

1. Create a FragmentEntryLink with invalid classNameid / classPK combo (remote or local service), but with a valid FragmentEntryId as a reference.
2. Check Usages for the Fragment Entry or Publish the Fragment Entry Link to live site

Expected Result
FragmentEntryLink not even allowed to be added with invalid classNameId/classPK in the first place

Actual Result
Because we tried fetching the FragmentEntryLink's className id / classPK and failed, we got an exception thrown

Tested on Tomcat 9.0.10 + MySQL 5.7
Portal master GIT ID: 826a5b06994aa6d1cd2f0afc79023ad7e44f627b