telepathyClient: fix incorrect usernameSupported setting

g_match_info_fetch(), used by polari_util_match_identify_message() to retrieve username from identify message, returns an empty string if no username was provided. Hence comparing the result against null is incorrect.

As the result, identify-username-supported key was always set to true, even if no username was passed in the identify message by the user. Consequently, that username was always used in the identify command issued by Polari, which in turn led to inability to authenticate in e.g. OFTC network thanks to its NickServ using a different parameters order (see #84).