Add length check for empty policy content

If an empty file used as input for policy creation a zero sized
byte array gets passed. Corrected the check to see length and if
it's zero, error out. Earlier error was confusing as below
```
mc: <ERROR> Unable to create new policy. You must provide the Content-Length HTTP header.
```

Signed-off-by: Shubhendu Ram Tripathi <[email protected]>

policy-commands.go

@@ -169,7 +169,7 @@ func (adm *AdminClient) RemoveCannedPolicy(ctx context.Context, policyName strin
 
 // AddCannedPolicy - adds a policy for a canned.
 func (adm *AdminClient) AddCannedPolicy(ctx context.Context, policyName string, policy []byte) error {
-	if policy == nil {
+	if len(policy) == 0 {
 		return ErrInvalidArgument("policy input cannot be empty")
 	}