Playing as Lilithe, If:
a) Grandmaster Variax was summoned and thus buffing your bloodborn spell to shadowspawn
a) shadowspawn is casted
b) but all nearby spaces are occupied by other minions/creatures
Already summoned/existing wraithlings are not given the +4/+4 buff.
The description of the shadowspawn bloodborn spell provided by Grandmaster Variax does not imply that the buffing all wraithling portion is dependent on the wraithling summoning being successful.