RPL_ISUPPORT: fix handling of empty string in PREFIX

Callers expect an array even if the string is empty, give them that.

Happened in thelounge when connecting to psyced.org... they return the following data:

{
    options: {
    CASEMAPPING: 'ascii',
    PREFIX: '',
    PSYC: '.99',
    ALIAS: true,
    AVAILABILITY: true,
    FRIEND: true,
    HISTORY: true,
    MOOD: true,
    SHOUT: true,
    SSET: true,
    STATUS: true,
    SUBSCRIBE: true,
    THREAD: true,
    TRUST: true,
    CHANTYPES: [ '#' ],
    CHANMODES: [ '' ],
    NICKLEN: '256',
    CHANNELLEN: '256',
    TOPICLEN: '444',
    KICKLEN: '444',
    AWAYLEN: '444',
    MAXTARGETS: '1',
    CHARSET: 'UTF-8',
    NETWORK: 'psyced.org',
    CTCP: 'PRESENCE,TS',
    UNIFORMS: 'psyc,xmpp',
    'ARE SUPPORTED BY THIS SERVER': true
  },
  cap: [],
  tags: {}
}

This makes irc-framework return {PREFIX: ''} (among other keys) which is undesired I guess

kiwiirc / irc-framework

RPL_ISUPPORT: fix handling of empty string in PREFIX #284