Fixes crash when plugin configuration is not defined in a job

[andreygolev]

If there's no plugin configuration in a job definition, dkron will crash with error:
panic: assignment to entry in nil map

This PR fixes it.

[vcastellm]

LGTM

[vcastellm]

I don't know why this test is failing

[vcastellm]

@andreygolev can you modify .github actions to this?

name: Test
on: [push,pull_request]
jobs:
  build:
    name: Test
    runs-on: ubuntu-latest
    steps:

    - name: Set up Go 1.14
      uses: actions/setup-go@v1
      with:
        go-version: 1.14
      id: go

    - name: Check out code into the Go module directory
      uses: actions/checkout@v2

    - name: Get dependencies
      run: go get -v -t -d ./...

    - name: Test without docker
      run: go test -v -timeout 200s ./...