Allow setting name of RequiresDirective to null

According to the child descriptor property, it's optional,
so it makes sense that you should be able to set it to null.
The Javadoc also doesn't mention anything about null not being allowed.

Signed-off-by: David Thompson <davthomp@redhat.com>

org.eclipse.jdt.core.tests.model/src/org/eclipse/jdt/core/tests/rewrite/describing/ASTRewritingModuleDeclarationTest.java

@@ -396,4 +396,30 @@ public void testBug542106_since_9() throws Exception {
 			if (javaProject != null) deleteProject(javaProject);
 		}
 	}
+	public void testSetModuleNameNull_since_9() throws Exception {
+		IJavaProject javaProject = null;
+		try {
+			javaProject = createProject("P_9", JavaCore.VERSION_9);
+			IPackageFragmentRoot currentSourceFolder = getPackageFragmentRoot("P_9", "src");
+			IPackageFragment pack1= currentSourceFolder.getPackageFragment(Util.EMPTY_STRING);
+			String content =
+					"module first {\n" +
+					"	requires module1;\n" +
+					"}";
+			ICompilationUnit cu= pack1.createCompilationUnit("module-info.java", content, false, null);
+			CompilationUnit astRoot= createAST(cu);
+			ASTRewrite rewrite= ASTRewrite.create(astRoot.getAST());
+			ModuleDeclaration moduleDeclaration = astRoot.getModule();
+			RequiresDirective requiresDirective = (RequiresDirective)moduleDeclaration.moduleStatements().get(0);
+			rewrite.remove(requiresDirective.getName(), null);
+			String preview= evaluateRewrite(cu, rewrite);
+			content =
+					"module first {\n" +
+					"	requires ;\n" +
+					"}";
+			assertEqualString(preview, content);
+		} finally {
+			if (javaProject != null) deleteProject(javaProject);
+		}
+	}
 }

org.eclipse.jdt.core/dom/org/eclipse/jdt/core/dom/RequiresDirective.java

@@ -199,17 +199,6 @@ public int getModifiers() {
 	 * @return the module referenced
 	 */
 	public Name getName()  {
-		if (this.name == null) {
-			// lazy init must be thread-safe for readers
-			synchronized (this) {
-				if (this.name == null) {
-					preLazyInit();
-					this.name = postLazyInit(
-							this.ast.newQualifiedName(new SimpleName(this.ast), new SimpleName(this.ast)),
-							NAME_PROPERTY);
-				}
-			}
-		}
 		return this.name;
 	}
 
@@ -224,9 +213,6 @@ public Name getName()  {
 	 * </ul>
 	 */
 	public void setName(Name name) {
-		if (name == null) {
-			throw new IllegalArgumentException();
-		}
 		ASTNode oldChild = this.name;
 		preReplaceChild(oldChild, name, NAME_PROPERTY);
 		this.name = name;

org.eclipse.jdt.core/dom/org/eclipse/jdt/internal/core/dom/rewrite/ASTRewriteAnalyzer.java

@@ -2417,8 +2417,11 @@ public boolean visit(RequiresDirective node) {
 			return doVisitUnchangedChildren(node);
 		}
 		int pos = getPosAfterToken(node.getStartPosition(), TerminalTokens.TokenNamerequires);
+		if (node.getName() != null) {
+			this.beforeRequiredSpaceIndex = node.getName().getStartPosition();
+		}
 		rewriteNodeList(node, RequiresDirective.MODIFIERS_PROPERTY, pos, String.valueOf(' '), String.valueOf(' '));
-		rewriteRequiredNode(node, RequiresDirective.NAME_PROPERTY);
+		rewriteNode(node, RequiresDirective.NAME_PROPERTY, this.beforeRequiredSpaceIndex, ASTRewriteFormatter.SPACE);
 		return false;
 	}