default_param can be null in network commands, but handlers assume not null

[janry]

Can be fixed by adding guard in each handler (265.patch) or by ensuring default_param is not null before dispatching command handler (266.patch).

I'm not sure if I covered every place in 265.patch - but doesn't matter much, because potential deficiencies will be detected and fixed later.

[prissi]

I personally prefer 266. I think that then one needs also to make sure that in local execution the default_param is not zero since locally commands may use the queue too.

[janry]

Hmm, it seems that 266 is wrong, because some handlers rely on default_param being null. For the same reason we can't replace nulls with empty string in local commands. One example is tool_build_house_t.

We could convert handlers that anticipate null param to anticipate "" param instead. And then we can replace nulls with "" on dispatch. I will check if that is viable.

[janry]

A patch that
1) replaces null-triggered behavior in command handlers to be ""-triggered instead
2) guards against null param on command dispatch for both network and local paths