Fix reading html content of a package fragment to not return null (#1657

)

- in the case of a package fragement, when we search for Javadoc
  content and do not find any package info, just return the empty
  string so Javadoc hover will at least show the package name

org.eclipse.jdt.core.manipulation/common/org/eclipse/jdt/core/manipulation/internal/javadoc/CoreJavadocAccess.java

@@ -184,8 +184,7 @@ protected String readHTMLContent(IPackageFragment packageFragment) throws CoreEx
 			return packageFragment.getAttachedJavadoc(null);
 
 		}
-
-		return null;
+		return ""; //$NON-NLS-1$
 	}