Correctly work when there are no labels

[SeanTAllen]

This fix comes from @jemc.

This script is using set -e so that errors won't go unnoticed.
But when commands are piped together, only the exit status of the
final command matters.

The grep command obviously fails when none of the tags are present
to find, which is causing the script to crash in that case. But it
seems that awk was always returning a success exit status even when
the grep failed, so that explains why we didn't see that issue when
the awk was the final command.

The true command returns success unconditionally and prints nothing.
So, adding || true has the desired effect of explicitly
ignoring/consuming the error from grep, while printing nothing into
the output stream of the subshell, so that the CHANGELOG_TYPES
variable will be an empty string in that case like we want.

Closes #9