@Override
			protected boolean shouldVisitNextChild(RuleNode node, String currentResult) {
				return currentResult == null || currentResult.isEmpty();
			}
This test verifies that {@link AbstractParseTreeVisitor#visitChildren} does not call
{@link org.antlr.v4.runtime.tree.ParseTreeVisitor#visit} after
{@link org.antlr.v4.runtime.tree.AbstractParseTreeVisitor#shouldVisitNextChild} returns
{@code false}.
runtime-testsuite/test/org/antlr/v4/test/runtime/java/api/TestVisitors.java