Fix for #22461 Empty ClassPath attribute in one or more classpath jars causes crash

[philwalk]

Change to how an empty or null jar manifest ClassPath: property is handled:

in dotty.tools.dotc.classpath.ClassPathFactory:

• classesInExpandedPath(...) returns and empty IndexedSeq rather than crash
• isJarOrZip returns false on a null reference rather than crash

in dotty.tools.dotc.classpath.FileUtils:

• createSourcePath fails with an error message on a null file parameter.

In the context of #22461, this causes an empty ClassPath: property to be treated the same as a mispelled or missing classpath entry, which are silently ignored, matching the behaviour of legacy scripts.

[som-snytt]

I was interested in trying it out because this is more-or-less shared code with Scala 2. I did not yet reproduce.

I would not expect to handle a possible null in more than one place. Dotty is usually complaining to me that such-and-such might be null, I have to sprinkle | Null if I think that is desired.