container: initialize results list

it solves:

$ podman ps --format=json
null

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

cmd/podman/shared/container.go

@@ -449,10 +449,8 @@ func GetPsContainerOutput(r *libpod.Runtime, opts PsOptions, filters []string, m
 // PBatch performs batch operations on a container in parallel. It spawns the
 // number of workers relative to the number of parallel operations desired.
 func PBatch(containers []*libpod.Container, workers int, opts PsOptions) []PsContainerOutput {
-	var (
-		wg        sync.WaitGroup
-		psResults []PsContainerOutput
-	)
+	var wg sync.WaitGroup
+	psResults := []PsContainerOutput{}
 
 	// If the number of containers in question is less than the number of
 	// proposed parallel operations, we shouldnt spawn so many workers.

test/e2e/ps_test.go

@@ -151,6 +151,13 @@ var _ = Describe("Podman ps", func() {
 		Expect(len(result.OutputToStringArray())).Should(BeNumerically(">", 0))
 	})
 
+	It("podman ps with no containers is valid json format", func() {
+		result := podmanTest.Podman([]string{"ps", "--format", "json"})
+		result.WaitWithDefaultTimeout()
+		Expect(result.ExitCode()).To(Equal(0))
+		Expect(result.IsJSONOutputValid()).To(BeTrue())
+	})
+
 	It("podman ps namespace flag with json format", func() {
 		_, ec, _ := podmanTest.RunLsContainer("test1")
 		Expect(ec).To(Equal(0))