Digdag CI fails on a pull request from a forked repository

[NomadBlacky]

https://github.com/NomadBlacky/digdag-plugin-datadog/runs/579377492?check_suite_focus=true

Run digdag secrets --local --set datadog.api_key=
  digdag secrets --local --set datadog.api_key=
  digdag secrets --local --set datadog.app_key=
  shell: /bin/bash -e {0}
  env:
    JAVA_HOME: /opt/hostedtoolcache/jdk/8.0.242/x64
    JAVA_HOME_8.0.242_x64: /opt/hostedtoolcache/jdk/8.0.242/x64
2020-04-11 17:21:16 +0000: Digdag v0.9.41
error: String index out of range: 0 (string index out of bounds)
##[error]Process completed with exit code 1.

Expected: Set keys from secrets of GitHub.

https://github.community/t5/GitHub-Actions/Allow-secrets-to-be-shared-with-forks-from-trusted-Actions/td-p/34278