artifact generation fails if there is no state-scripts input path

[KevinRohn]

Description:

The input parameter state-scripts is an optional parameter, but if the parameter is not set the build fails.

Error message:

Artifact name devicemigration-v2.8 is set for mender-artifact creation.
Device type raspberrypi3 is set for mender-artifact creation.
Mender module type deb is set for mender-artifact creation.
Artifact content dist/artifact-content path is set for mender-artifact creation.
Output path dist/artifact-output is set for mender-artifact creation.
Software name devicemigration is set for mender-artifact creation.
Software version v2.8 is set for mender-artifact creation.
Invalid script name: "entrypoint.sh". Scripts must have a name on the form: <STATE_NAME>_<ACTION>_<ORDERING_NUMBER>_<OPTIONAL_DESCRIPTION>. For example: 'Download_Enter_05_wifi-driver' is a valid script name.