Don't send empty username and password

[jackwilsdon]

Since 70a247c changed how we generate options, we now end up sending an empty username and password when they are not set, instead of sending no username and password. This ends up confusing some MQTT servers, which don't properly detect an empty username/password as an anonymous login.

This can be seen most clearly in the connect flags:

b5b3814 (one commit before 70a247c)

Connect Flags: 0x02, QoS Level: At most once delivery (Fire and Forget), Clean Session Flag
    0... .... = User Name Flag: Not set
    .0.. .... = Password Flag: Not set
    ..0. .... = Will Retain: Not set
    ...0 0... = QoS Level: At most once delivery (Fire and Forget) (0)
    .... .0.. = Will Flag: Not set
    .... ..1. = Clean Session Flag: Set
    .... ...0 = (Reserved): Not set

70a247c

Connect Flags: 0xc2, User Name Flag, Password Flag, QoS Level: At most once delivery (Fire and Forget), Clean Session Flag
    1... .... = User Name Flag: Set
    .1.. .... = Password Flag: Set
    ..0. .... = Will Retain: Not set
    ...0 0... = QoS Level: At most once delivery (Fire and Forget) (0)
    .... .0.. = Will Flag: Not set
    .... ..1. = Clean Session Flag: Set
    .... ...0 = (Reserved): Not set

Setting the options to null when they're false-y seems to fix this.

[ccvca]

Just some more background information, as I just tracked down the same Problem (#1207)

It may be also possible to fix this in mqtt-packet here:

https://github.com/mqttjs/mqtt-packet/blob/2dcc70c88f0b8b146b63f017f816568fdc453ebf/writeToStream.js#L259-L265

  // Username and password
  if (username != null) {
    writeStringOrBuffer(stream, username)
  }
  if (password != null) {
    writeStringOrBuffer(stream, password)
  }

[zambujal]

My mqtt addon mosquito frezes and then nothing maybe because of this? not even showing errors... tried several updates... so this is just bugs...