fix: ensure we can disable setting the rev-env-var

tests/unit/10_git.bats

@@ -68,7 +68,7 @@ teardown() {
   run bash -c "dokku config:get $TEST_APP GIT_REV"
   echo "output: "$output
   echo "status: "$status
-  assert_output_empty
+  assert_output ""
 }
 
 @test "(git) customize the GIT_REV environment variable" {

tests/unit/test_helper.bash

@@ -77,12 +77,6 @@ assert_output_exists() {
   [[ -n "$output" ]] || flunk "expected output, found none"
 }
 
-# ShellCheck doesn't know about $output from Bats
-# shellcheck disable=SC2154
-assert_output_empty() {
-  [[ -z "$output" ]] || flunk "expected no output, found $output"
-}
-
 # ShellCheck doesn't know about $output from Bats
 # shellcheck disable=SC2154
 assert_output_contains() {