Admin bar items without hrefs should be printed without links

[koopersmith]

In some cases, it is desirable to print an admin bar item without a link (instead of overriding the item output entirely).

A couple example use cases:

1. Empty dropdown headers.
2. Printing an inline search box (as we do on the front end).

[ocean90]

Duplicate/Related: #15519, #19149 and #19164

[koopersmith]

In [19328]:

Allow linkless items in the admin bar. Improve non-jQuery admin bar tabbing. fixes #19277, #19149, #19164, #15519. see #18197, #19088.

[koopersmith]

In [19499]:

Print title attributes for linkless toolbar items. see #19277.