Avoid errors from selecting "no selection" on multilink (issue2550722). As discussed in issue 2550722 there are various cases where selecting "no selection" on a multilink can result in inappropriate errors from Roundup: * If selecting "no selection" produces a null edit (a value was set in the multilink in an edit with an error, then removed again, along with all other changes, in the next form submission), so the page is rendered from the form contents including the "-<id>" value for "no selection" for the multilink. * If creating an item with a nonempty value for a multilink has an error, and the resubmission changes that multilink to "no selection" (and this in turn has subcases, according to whether the creation then succeeds or fails on the resubmission, which need fixes in different places in the Roundup code). All of these cases have in common that it is expected and OK to have a "-<id>" value for a submission for a multilink when <id> is not set in that multilink in the database (because the original attempt to set <id> in that multilink had an error), so the hyperdb.py logic to give an error in that case is thus removed. In the subcase of the second case where the resubmission with "no selection" has an error, the templating code tries to produce a menu entry for the "-<id>" multilink value, which also results in an error, hence the templating.py change to ignore such values in the list for a multilink.

Debugging Aids
--------------

Try turning on logging of DEBUG level messages. This may be done a number
of ways, depending on what it is you're testing:

1. If you're testing the database unit tests, then set the environment
   variable ``LOGGING_LEVEL=DEBUG``. This may be done like so:

    LOGGING_LEVEL=DEBUG python run_tests.py

   This variable replaces the older HYPERDBDEBUG environment var.

2. If you're testing a particular tracker, then set the logging level in
   your tracker's ``config.ini``.

SENDMAILDEBUG
=============

Set to a filename and roundup will write each email message
that it sends to that file instead to the internet. 
This environment variable is independent of the python -O flag.