Null result in echo response when echo request is with empty params array

[hsinyishen]

When the peer sends echo request with empty params array:
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 7b 22 69 64 22 3a 22 65 63 68 6f 22 2c 22 6d 65 |{"id":"echo","me|
|00000010| 74 68 6f 64 22 3a 22 65 63 68 6f 22 2c 22 70 61 |thod":"echo","pa|
|00000020| 72 61 6d 73 22 3a 5b 5d 7d |rams":[]} |
+--------+-------------------------------------------------+----------------+

The library constructs null result in response message:
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 7b 22 72 65 73 75 6c 74 22 3a 6e 75 6c 6c 2c 22 |{"result":null,"|
|00000010| 65 72 72 6f 72 22 3a 6e 75 6c 6c 2c 22 69 64 22 |error":null,"id"|
|00000020| 3a 22 65 63 68 6f 22 7d |:"echo"} |
+--------+-------------------------------------------------+----------------+

This causes peer disconnecting and reconnecting continuously due to protocol error.

[hechaoli]

Please apply this patch or wait for the next patch release.