Print headers for system connection ls

Print out the headers even if the system connection list
is empty to match the behavior of other list commands.

Signed-off-by: Urvashi Mohnani <umohnani@redhat.com>

test/e2e/system_connection_test.go

@@ -236,7 +236,7 @@ var _ = Describe("podman system connection", func() {
 			session := podmanTest.Podman(cmd)
 			session.WaitWithDefaultTimeout()
 			Expect(session).Should(Exit(0))
-			Expect(session.Out.Contents()).Should(BeEmpty())
+			Expect(len(session.OutputToStringArray())).Should(Equal(1))
 			Expect(session.Err.Contents()).Should(BeEmpty())
 		})
 	})

test/system/272-system-connection.bats

@@ -50,7 +50,8 @@ function _run_podman_remote() {
 # Very basic test, does not actually connect at any time
 @test "podman system connection - basic add / ls / remove" {
     run_podman system connection ls
-    is "$output" "" "system connection ls: no connections"
+    is "$output" "Name        URI         Identity    Default" \
+       "system connection ls: no connections"
 
     c1="c1_$(random_string 15)"
     c2="c2_$(random_string 15)"