HHChannel: uninitialized gates show up in `wildcardFind` and cause silent crash when accessed

[subhacom]

Even when any of the Xpower/Ypower/Zpower is not set, so that the actual gate object does not exist, all three show up as children of the HHChannel.
One can access the child gate with moose.element(f'{channel.path}/{gateName}'). One can even access its name field. But attempt to access any other field results in a silent crash.

Example for reproducing:

In [1]: import moose

In [2]: ch = moose.HHChannel('ch')

In [3]: moose.le(ch)
Elements under /ch[0]
    /ch[0]/gateX
    /ch[0]/gateY
    /ch[0]/gateZ

Out[3]: ['/ch[0]/gateX', '/ch[0]/gateY', '/ch[0]/gateZ']

In [4]: gate = moose.element(f'{ch.path}/gateX')

In [5]: gate
Out[5]: <moose.HHGate id=487 dataIndex=0 path=/ch[0]/gateX[0]>

In [6]: gate.name
Out[6]: 'gateX'

In [7]: gate.min