update type null for dependency upgrade

[pexa-slee]

Hi,

We have been using this action quite heavily for our auto-merge reusable workflow, which has been working very well.

We came across a PR today where the update-type was null. The previous-version and new-version were also empty.

The version of the action is dependabot/fetch-metadata@v2

The PR title was:
NOJIRA | Dependabot | Bump io.netty:netty-common from 4.1.115.Final to 4.1.118.Final in /xxx_core in the maven group

The change itself was Updates io.netty:netty-common from 4.1.115.Final to 4.1.118.Final

Is this down to the syntax?

The actual commit msg details:

NOJIRA | Dependabot | Bump io.netty:netty-common
Bumps the maven group in /xxx_core with 1 update: [io.netty:netty-common](https://github.com/netty/netty).


Updates `io.netty:netty-common` from 4.1.115.Final to 4.1.118.Final
- [Commits](https://github.com/netty/netty/compare/netty-4.1.115.Final...netty-4.1.118.Final)

---
updated-dependencies:
- dependency-name: io.netty:netty-common
  dependency-type: direct:production
  dependency-group: maven
...

Signed-off-by: dependabot[bot] <support@github.com>

Thanks,
Shane