Tabs with empty tabs (e.g. created from clipboard) cause problems with links handling

[Lerm]

If JD GUI has some tabs with empty path (not bound to real files - e.g. tabs created from the clipboard), then they will prevent navigating to the right tab while clicking on the type link.

Test case:
0. Start JD-GUI.

1. Open rt.jar from JDK in JD-GUI.
2. Copy following text and paste it into JD-GUI - this should create new tab (e.g. 'clipboard-1.log') with such text:
   `` at java.lang.String.equals(String.class)
3. Click on 'java.lang.String.equals' link in this newly created tab.
   ER: JD-GUI is navigated to the 'java/lang/String' class and 'equals' method in 'rt.jar' tab.
   AR: JD-GUI stays at the current tab ('clipboard-1.log') or navigates to the last tab created from clipboard instead of navigating to 'rt.jar'.

The problem seems to be related to the method TabbedPanel.showPage - it iterates through all existing tabs, checking whether tab path is at the beginning of the resolved URL path. However, tabs created from the clipboard have empty path, so they always match this check.