doc: use nullish instead of falsy

Used nullish instead of falsy to point at that
the parameter now only accepts a `string`,
`null` or `undefined`.

doc/api/dgram.md

@@ -521,7 +521,7 @@ If `msg` is an array, `offset` and `length` must not be specified.
 
 The `address` argument is a string. If the value of `address` is a host name,
 DNS will be used to resolve the address of the host. If `address` is not
-provided or otherwise falsy, `'127.0.0.1'` (for `udp4` sockets) or `'::1'`
+provided or otherwise nullish, `'127.0.0.1'` (for `udp4` sockets) or `'::1'`
 (for `udp6` sockets) will be used by default.
 
 If the socket has not been previously bound with a call to `bind`, the socket