Restrict system library check to require Java nature

[ptziegler]

The validation of the project classpath introduced with 74d062b always fails, if the project doesn't have a Java nature (and thus usually no .classpath file).

This causes issues when e.g. trying to open a source file with the Java editor or simply when trying to use the AST parser outside of a Java project. With this change, the check becomes more conservative and only fails, if both the Java nature exists and no system library is configured.

Note that one can avoid this exception by disabling the binding resolution. But given that this requires changes in user code, it is more prudent to only fail in cases where this is the desired behavior.

Resolves #3298

Author checklist

• I have thoroughly tested my changes
• The change is following the coding conventions
• I have signed the Eclipse Contributor Agreement (ECA)

[ptziegler]

I've added an additional test case that also checks the situation where the AST parser is used without any project.

Note that this currently causes testGH3047() to fail because the source file is parsed without the Java project being set (and also because no requestor is set). This hasn't thrown an exception up until now and therefore, at least from my perspective, shouldn't.

[ptziegler]

After looking closer testGH3047() and testGH3047_2() are effectively copies of one another, except that the latter is passing the IJavaProject to the ASTParser. The problem is that testGH3047() is effectively the setup we have in our application and what's causing the regression.