conformance: skip check (by default) for zero layers in image manifest

As per image-spec, since this is a SHOULD (hence recommended) and now
turned off by default.

https://raw.githubusercontent.com/opencontainers/image-spec/main/manifest.md
===

    layers array of objects

    Each item in the array MUST be a descriptor. For portability, layers
    SHOULD have at least one entry.

===

One can enable this test by setting env var:
OCI_SKIP_EMPTY_LAYER_PUSH_TEST=0

Signed-off-by: Ramkumar Chinchani <rchincha@cisco.com>

conformance/setup.go

@@ -309,7 +309,7 @@ func init() {
 	runPushSetup = true
 	runContentDiscoverySetup = true
 	runContentManagementSetup = true
-	skipEmptyLayerTest = false
+	skipEmptyLayerTest = true
 	deleteManifestBeforeBlobs = false
 
 	if os.Getenv(envVarTagName) != "" &&
@@ -322,7 +322,9 @@ func init() {
 		runContentDiscoverySetup = false
 	}
 
-	skipEmptyLayerTest, _ = strconv.ParseBool(os.Getenv(envVarPushEmptyLayer))
+	if v, ok := os.LookupEnv(envVarPushEmptyLayer); ok {
+		skipEmptyLayerTest, _ = strconv.ParseBool(v)
+	}
 	deleteManifestBeforeBlobs, _ = strconv.ParseBool(os.Getenv(envVarDeleteManifestBeforeBlobs))
 	automaticCrossmountVal := ""
 	automaticCrossmountVal, runAutomaticCrossmountTest = os.LookupEnv(envVarAutomaticCrossmount)