Just switch the order:
z = dict(d2.items() + d1.items())
By the way, you may also be interested in the potentially faster update
method.
In Python 3, you have to cast the view objects to lists first:
z = dict(list(d2.items()) + list(d1.items()))
If you want to special-case empty strings, you can do the following:
def mergeDictsOverwriteEmpty(d1, d2):
res = d2.copy()
for k,v in d2.items():
if k not in d1 or d1[k] == '':
res[k] = v
return res