[develop] Fix build breaking when specifying commit hash

[scottarnette]

Change Description

Fixed issue where specifying a commit for eosio or eosio.cdt in pipeline.jsonc would result in the build to fail.

• This was due to the checks for EOSIO_COMMIT or CDT_COMMIT resulting in null strings when checking the GitHub API.
• We took this value and piped into grep -v 'null'. This resulted in an exit code of 1 as no strings exist that do not match null.
• This solution now replaces null with empty lines, then removes all empty lines to either get an empty string or the commit HEAD of the branch/tag as determined by GitHub's API. If the string is empty, it means we use the value supplied in pipeline.jsonc as originally intended.

See:
Build #602 | Buildkite build where the CDT version is specified by a commit hash.
Build #304 | Travis build where the CDT version is specified by a commit hash.

Deployment Changes

• Deployment Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions