Disallow IPackageFragmentRoot from being null within ProjectTypeContainers

In all current use cases of this internal class the value passed in
was non-null anyway, so there is no functional change with this commit,
it simply prevents future changes from trying to pass in a null here

An extra complication is that in 02fa168
the javadoc was updated to allow null IPackageFragmentRoot but the code
did not match that.

Fixes eclipse-pde#2202 (comment)

Author: jonahgraham

apitools/org.eclipse.pde.api.tools/src/org/eclipse/pde/api/tools/internal/model/ProjectTypeContainer.java

@@ -15,6 +15,7 @@
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Objects;
 import java.util.Set;
 import java.util.SortedSet;
 import java.util.TreeSet;
@@ -62,15 +63,13 @@ public class ProjectTypeContainer extends ApiElement implements IApiTypeContaine
 	 * @param parent the {@link IApiElement} parent for this container
 	 * @param container folder in the workspace
 	 * @param packageFragmentRoot optional package fragment root for JDT-based
-	 *            package discovery, may be <code>null</code>
+	 *            package discovery
 	 * @since 1.3.300
 	 */
 	public ProjectTypeContainer(IApiElement parent, IContainer container, IPackageFragmentRoot packageFragmentRoot) {
 		super(parent, IApiElement.API_TYPE_CONTAINER, container.getName());
 		this.fRoot = container;
-		if (packageFragmentRoot != null) {
-			this.fPackageFragmentRoots.add(packageFragmentRoot);
-		}
+		this.fPackageFragmentRoots.add(Objects.requireNonNull(packageFragmentRoot));
 	}
 
 	/**
@@ -81,8 +80,9 @@ public ProjectTypeContainer(IApiElement parent, IContainer container, IPackageFr
 	 * @since 1.3.400
 	 */
 	public void addPackageFragmentRoot(IPackageFragmentRoot root) {
-		if (root != null && !fPackageFragmentRoots.contains(root)) {
-			fPackageFragmentRoots.add(root);
+		IPackageFragmentRoot nonNullRoot = Objects.requireNonNull(root);
+		if (!fPackageFragmentRoots.contains(nonNullRoot)) {
+			fPackageFragmentRoots.add(nonNullRoot);
 			// Clear cached package names so they will be recomputed
 			fPackageNames = null;
 		}