Matrix builds requiring at least two variables is a unnecessary limitation

Summary

Trying out the new matrix build feature, and its great! but come across a what seems like a strange limitation.

jobs:build_prod:parallel:matrix:variables config requires at least 2 items

Steps to reproduce

This is my job definition, so this TYPE variable has 4 variants, but GitLab refuses this configuration

build_prod:
  parallel:
    matrix:
      - TYPE: ['fpm', 'console', 'console-super', 'nginx']
  script:
    - gcloud builds submit --substitutions _TYPE=$TYPE

### Work around

I can add a dummy variable, and now I get the correct config, with 4 builds being run.

build_prod:
  parallel:
    matrix:
      - TYPE: ['fpm', 'console', 'console-super', 'nginx']
        DUMMY: 'dummy'
  script:
    - gcloud builds submit --substitutions _TYPE=$TYPE