initrd: fix generating default BOOTIF= connection
There is a bug when parsing a BOOTIF= without any existing connection. The generated connection doesn't have wired setting and later we try to access it:
# nm-initrd-generator --stdout -- BOOTIF=01-50-50-00-9f-21-21
(nm-initrd-generator:1546): libnm-CRITICAL **: ((libnm-core/nm-setting-wired.c:205)): assertion '<dropped>' failed
(nm-initrd-generator:1546): GLib-GObject-CRITICAL **: g_object_set: assertion 'G_IS_OBJECT (object)' failed
Fix this.