This change fixes a NPE in EclipseCompiler, when passing an empty
iterable for the "classes" argument of JavaCompiler.getTask().

Additionally, fixed NPE in error handling if the diagnosticListener is
null (which is valid use case).

See https://docs.oracle.com/en/java/javase/11/docs/api/java.compiler/javax/tools/JavaCompiler.html#getTask(java.io.Writer,javax.tools.JavaFileManager,javax.tools.DiagnosticListener,java.lang.Iterable,java.lang.Iterable,java.lang.Iterable)

Change-Id: I9070a533c7931d4c896dd50a46b5df0fd41f741e
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.core/+/188388
Tested-by: JDT Bot <jdt-bot@eclipse.org>
Reviewed-by: Andrey Loskutov <loskutov@gmx.de>