* lisp/org-duration.el (org-duration-to-minutes): Accept the empty
string.
* testing/lisp/test-org-duration.el (test-org-duration/to-minutes):
Add test.
For backward compatibility with `org-duration-string-to-minutes'
accept the empty string as a duration.
* lisp/org-duration.el (org-duration-to-minutes): Fix bug where the
same unit with a decimal unit would be matched multiple times.
* testing/lisp/test-org-duration.el (test-org-duration/to-minutes):
Fix test.