Add defensive null check to stop null pointer exception

[Mustard]

When joining tables (entities) that have a @customizer (history policy) eclipselink will throw a NullPointerException

I have a simple proof of concept project that demonstrates the issue. Ideally this could be added to the eclipselink project as a test, any help around where to add this test in the project would be appreciated as I couldn't even get the 2.7 branch to build locally and am unfamiliar with the project

This pull request would resolve this issue https://bugs.eclipse.org/bugs/show_bug.cgi?id=526836

A possible other solution / improvement might be to initialise asOfClause in org.eclipse.persistence.internal.expressions.DataExpression such that it is not null in the first place

protected AsOfClause asOfClause = AsOfClause.NO_CLAUSE;

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[lukasj]

wrt test - take a look at jpa/eclipselink.jpa.testapps and try to find an existing test this could be added to - jpa.test.advanced or jpa.test.jpql look like good candidates; it does not make much sense to add whole new project for this

[Mustard]

Hi @lukasj

Thanks for your help!

Took me a while to get back to it but I have added tests to the pull request, if there is anything else I can do to move this along let me know