Win64, Blender 2.79 / 2.8

Type in python console:

Quaternion()

Result:

Quaternion((0.0, 0.0, 0.0, 0.0))

Correct result ( Identity quaternion (w value must be 1.0 ) :

Quaternion((1.0, 0.0, 0.0, 0.0))

Even docstring says:

(), *no args*
    Create an identity quaternion

git point of interest: https://git.blender.org/gitweb/gitweb.cgi/blender.git/blob/HEAD:/source/blender/python/mathutils/mathutils_Quaternion.c#l1101