fix push check image name is not empty
fix push check image name is not empty

Close #1708

This PR check the input image name is not a nil string if both image name and registry are provided. Avoid triggering go segmentation violation later in the code.

Signed-off-by: Qi Wang <qiwan@redhat.com>

Closes: #1717
Approved by: vrothberg

TomSweeneyRedHat/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,rhatdan/buildah,nalind/buildah-1,nalind/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,rhatdan/buildah,projectatomic/buildah,nalind/buildah,nalind/buildah-1,rhatdan/buildah