Hi,

When the environment variable "DISPLAY" is invalid, exporting plantuml block in org will got error message:
Exception in thread "main" java.awt.AWTError: Can't connect to X11 window server using 'localhost:11.0' as the value of the DISPLAY variable.
    at sun.awt.X11GraphicsEnvironment.initDisplay(Native Method)
...
To reproduce this issue, install the plantuml.jar and make sure it work inside org-mode.
Then open `/tmp/testing.org` and  input follow plantuml code and eval-expression `(progn (setenv "DISPLAY" "localhost:90")(org-html-export-to-html))`.
The root cause is org-mord execute the plantuml.jar in command line without headless parameter, then plantuml will try read the DISPLAY, it's unnecessary.
(For standalone plantuml, `org-plantuml-executable-args` has default value '-headless' to avoid reading DISPLAY).

Please help review the patch. Thanks.

Best Regards
Lin