config: accept env vars set to "", discern from unset

Fixes #7831 to a certain extent.

Signed-off-by: Stephan Renatus <stephan@styra.com>

internal/config/config.go

@@ -138,7 +138,8 @@ func subEnvVars(s string) string {
 		// Lookup the variable in the environment. We do not
 		// play by bash rules: if its undefined we'll keep it
 		// as-is, it could be replaced somewhere down the line.
-		if lu := os.Getenv(varName); lu != "" {
+		// If it's set to "", we'll return that.
+		if lu, ok := os.LookupEnv(varName); ok {
 			return lu
 		}
 		return s

internal/config/config_test.go

@@ -96,7 +96,7 @@ func TestSubEnvVarsVarsSubMissingEnvVar(t *testing.T) {
 	envKey := setTestEnvVar(t, "var1", "foo")
 	configYaml := fmt.Sprintf("field1: '${%s}'", envKey)
 
-	// Remove the env var and expect the system to sub in ""
+	// Remove the env var and expect the system to keep the key as-is
 	os.Unsetenv(envKey)
 	expected := configYaml // untouched
 
@@ -118,6 +118,20 @@ func TestSubEnvVarsVarsSubEmptyVarName(t *testing.T) {
 	}
 }
 
+func TestSubEnvVarsVarsSubEmptyEnvVar(t *testing.T) {
+	envKey := setTestEnvVar(t, "var1", "foo")
+	configYaml := fmt.Sprintf("field1: '${%s}'", envKey)
+
+	t.Setenv(envKey, "")
+	expected := "field1: ''"
+
+	actual := subEnvVars(configYaml)
+
+	if actual != expected {
+		t.Errorf("Expected: '%s'\nActual: '%s'", expected, actual)
+	}
+}
+
 func TestMergeValuesNoOverride(t *testing.T) {
 	dest := map[string]any{}
 	src := map[string]any{