Group Types: the bp_group_type_list() template tag can output empty links

[imath]

If the Group Type's has_directory property is set to false and its show_in_list property is true, the Group type's directory url is empty and the template tag is using it into the href attribute of the link of the listed Group Type. Clicking on it, we are refreshing the current group's page.

Moreover, using the Group Type's name property into the text of the link is wrong as it's not the singular name, it's the plural one.

Moreover II, The label used before the group types list should only be plural if there are more than one Group Type listed

[imath]

​8401.patch is addressing the issues.

[imath]

In 12801:

Group Types: improve the bp_group_type_list() template tag

• Make sure to avoid displaying empty links if the Group Types object’s has_directory property is set to false.
• To improve consistency with the Groups directory message informing it is filtered according to a Group Type, use singular Group Type names for the texts used into the list.
• Edit The $r['label'] argument of the bp_get_group_type_list() function so that it accepts an array containing the plural & singular labels to use according to the Group's number of group types it is assigned to.
• Ensure backward compatibility in case developers are still using a string for this $r['label'] argument.

Fixes #8401