API: containers/json always set application/json content type

When we return no containers we just return `[]` but we still have to keep
the content type header `application/json` so external tools can correctly
parse the output.

Fixes containers#14647

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

pkg/api/handlers/libpod/containers.go

@@ -115,10 +115,6 @@ func ListContainers(w http.ResponseWriter, r *http.Request) {
 		utils.InternalServerError(w, err)
 		return
 	}
-	if len(pss) == 0 {
-		utils.WriteResponse(w, http.StatusOK, "[]")
-		return
-	}
 	utils.WriteResponse(w, http.StatusOK, pss)
 }
 

test/apiv2/20-containers.at

@@ -16,7 +16,12 @@ podman pull $ENV_WORKDIR_IMG &>/dev/null
 # Ensure clean slate
 podman rm -a -f &>/dev/null
 
-t GET "libpod/containers/json (at start: clean slate)" 200 length=0
+t GET "libpod/containers/json (at start: clean slate)" 200 \
+  "[]" \
+  length=0
+# check content type: https://github.com/containers/podman/issues/14647
+response_headers=$(cat "$WORKDIR/curl.headers.out")
+like "$response_headers" ".*Content-Type: application/json.*" "header does not contain application/json"
 
 # Regression test for #12904 (race condition in logging code)
 mytext="hi-there-$(random_string 15)"