tests: MWInternalLinkAnnotation getFragment

Also, fix docs for it. It could also return null for a missing fragment.

Change-Id: Ib6df379c5c3686a59026b6b66f26b3050ede231a

modules/ve-mw/dm/annotations/ve.dm.MWInternalLinkAnnotation.js

@@ -204,12 +204,12 @@ ve.dm.MWInternalLinkAnnotation.static.getLookupTitle = function ( original ) {
  *
  * @static
  * @param {string|mw.Title} original Original title
- * @return {string} Fragment for the title, or an empty string if it was invalid
+ * @return {string|null} Fragment for the title, or null if it was invalid or missing
  */
 ve.dm.MWInternalLinkAnnotation.static.getFragment = function ( original ) {
 	var title = original instanceof mw.Title ? original : mw.Title.newFromText( original );
 	if ( !title ) {
-		return '';
+		return null;
 	}
 	return title.getFragment();
 };

modules/ve-mw/tests/dm/ve.dm.MWInternalLinkAnnotation.test.js

@@ -61,3 +61,50 @@ QUnit.test( 'toDataElement', function ( assert ) {
 		assert.deepEqual( ve.dm.MWInternalLinkAnnotation.static.toDataElement( [ cases[ i ].element ], converter ), cases[ i ].expected, cases[ i ].msg );
 	}
 } );
+
+QUnit.test( 'getFragment', function ( assert ) {
+	var	i, l,
+		cases = [
+			{
+				msg: 'No fragment returns null',
+				original: 'Foo',
+				expected: null
+			},
+			{
+				msg: 'Invalid title returns null',
+				original: 'A%20B',
+				expected: null
+			},
+			{
+				msg: 'Blank fragment returns empty string',
+				original: 'Foo#',
+				expected: ''
+			},
+			{
+				msg: 'Extant fragment returns same string',
+				original: 'Foo#bar',
+				expected: 'bar'
+			},
+			{
+				msg: 'Hash-bang works returns full string',
+				original: 'Foo#!bar',
+				expected: '!bar'
+			},
+			{
+				msg: 'Double-hash returns everything after the first hash',
+				original: 'Foo##bar',
+				expected: '#bar'
+			},
+			{
+				msg: 'Multi-fragment returns everything after the first hash',
+				original: 'Foo#bar#baz#bat',
+				expected: 'bar#baz#bat'
+			}
+		];
+
+	QUnit.expect( cases.length );
+
+	for ( i = 0, l = cases.length; i < l; i++ ) {
+		assert.deepEqual( ve.dm.MWInternalLinkAnnotation.static.getFragment( cases[ i ].original ), cases[ i ].expected, cases[ i ].msg );
+	}
+} );