`network.merge(...)` seems to add default values for outputs

Version Checks (indicate both or one)

• I have confirmed this bug exists on the lastest release of PyPSA.

• I have confirmed this bug exists on the current master branch of PyPSA.

Issue Description

Not 100% on this one.

Calling n_merged = n.merge(n_other) seems to restore some values to their default values, even if they were not present in either network.

In my case, n and n_other had both <n|n_other.>c.links.dynamic["p0"].empty == True.
However then n_mergedc.links.dynamic["p0"].empty == False, with all valeus set to 0.

This is not bad per-se. The problem is in combination with other functionality that determines if a network has already been solved or not and behaves differently depending on which attributes are set / not set.
In my case that is the network.cluster.temporal.segmentation(...) function that throws a warning as it encounters values in p1 and others:

https://github.com/PyPSA/PyPSA/blob/2e22ebd6bd8315a5e6c51f7581cbb56aa7f1e4af/pypsa/clustering/temporal.py#L28-L41

Reproducible Example

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Expected Behavior

n.merge(...) should not re-add attributes with their default values if they were not present in the originating networks.

Installed Versions