Remove no longer needed handling of null ProBError.getErrors

If there is no detailed error information, it now always returns an
empty list and never null.

src/main/java/de/prob2/jupyter/ProBKernel.java

@@ -902,12 +902,6 @@ public final class ProBKernel extends BaseKernel {
 					this.errorStyler.primary("Error from ProB: ")
 					+ this.errorStyler.secondary(String.valueOf(proBError.getOriginalMessage()))
 				).split("\n")));
-				if (proBError.getErrors() == null) {
-					// If the errors list is null rather than empty, don't show the "no error messages" message.
-					// (This matches the normal behavior of ProBError.)
-				} else if (proBError.getErrors().isEmpty()) {
-					out.addAll(this.errorStyler.primaryLines("ProB returned no error messages.\n"));
-				} else {
 				if (proBError.getErrors().size() > 1) {
 					out.addAll(this.errorStyler.primaryLines(proBError.getErrors().size() + " errors:\n"));
 				}
@@ -917,7 +911,6 @@ public final class ProBKernel extends BaseKernel {
 						out.addAll(formatErrorSource(contextSourceLines, location));
 					}
 				}
-				}
 				return out;
 			} else {
 				return this.errorStyler.secondaryLines(e.toString());