TLorentzVectorArray.p3 and TLorentzVectorArray.boostp3 broken, TLorentzVectorArray.empty_like returns invalid

[jrueb]

Calling empty_like on a TLorentzVectorArray gives invalid results. The returning object's _valid method raises exceptions.
Example:

import awkward
import uproot_methods

a = awkward.fromiter([[1, 2], [3]])
t = uproot_methods.TLorentzVectorArray.from_cartesian(a, a, a, a * 10)
print(t.empty_like())

This causes p3 and boostp3 to break, however it might not be visible in all cases. Slicing for example will make it visible:

t[[True, True]].p3

raises an exception.