use sensible defaults for the git provider when no data is found

[jgangemi]

INFO[0147] [stage=deploy] ERRO took 2m25.238s
INFO[0147] [stage=deploy] Error: Invalid template interpolation value
INFO[0147] [stage=deploy] 
INFO[0147] [stage=deploy]   on /builds/xxx/.togomak/pipelines/tmp/e29e2611-1ea6-4fa4-b002-79788588f2302885812944/deploy/publish.hcl line 81, in stage "echo":
INFO[0147] [stage=deploy]   79:   script = <<EOT
INFO[0147] [stage=deploy]   80:   echo "branch=${data.env.branch.value}"
INFO[0147] [stage=deploy]   81:   echo "commits_since_last_tag=${data.git.this.commits_since_last_tag}"
INFO[0147] [stage=deploy]   82:   echo "last_tag=${data.git.this.last_tag}"
INFO[0147] [stage=deploy]   83:   echo "timestamp=${local.timestamp}"
INFO[0147] [stage=deploy]   84:   echo "gcp: $GOOGLE_APPLICATION_CREDENTIALS"
INFO[0147] [stage=deploy]   85:   EOT
INFO[0147] [stage=deploy] 
INFO[0147] [stage=deploy] with data.git.this.commits_since_last_tag set to null.
INFO[0147] [stage=deploy] 
INFO[0147] [stage=deploy] The expression result is null. Cannot include a null value in a string template.
INFO[0147] [stage=deploy] 
ERRO[0147] <nil>: failed to run command (stage.deploy); exit status 1

if fields don't exist, return sensible defaults instead of null.