Unjustified consistency check warnings with multi-input/-output links
In a network allowing for n:1 or 1:m links (links with multiple input or output buses) `network.consistency_check()` throws warnings if there exist links in the network which do not use all input/output buses.

E.g.:
* the `link` component is extended to support `bus0` to `bus2`
* if a link only uses `bus0` and `bus1`
* then `network.consistency_check()` throws the following warning:

```
2022-11-17 08:09:08 pypsa.components WARNING  The following links have bus2 which are not defined:
```

## Checklist

- [x] I am using the current [`master`](https://github.com/pypsa/pypsa/tree/master) branch or the latest [release](https://github.com/pypsa/pypsa/releases). Please indicate:

## Describe the Bug

In earlier versions of `consistency_check()` only the correct setting of `bus0` and `bus1` was evaluated.
In the newer version there seems to be no way to suppress the warning despite the link having intentionally no `bus2` connection.

## Error Message

```
2022-11-17 08:09:08 pypsa.components WARNING  The following links have bus2 which are not defined:
```

diff --git a/pypsa/components.py b/pypsa/components.py
index 32263d32..03987e57 100644
--- a/pypsa/components.py
+++ b/pypsa/components.py
@@ -1327,6 +1327,10 @@ class Network(Basic):
         for c in self.iterate_components():
             for attr in bus_columns(c.df):
                 missing = ~c.df[attr].isin(self.buses.index)
+                # if bus2, bus3... contain empty strings do not warn
+                if c.name in self.branch_components:
+                    if int(attr[-1]) > 1:
+                        missing &= c.df[attr] != ""
                 if missing.any():
                     msg = "The following %s have %s which are not defined:\\n%s"
                     logger.warning(msg, c.list_name, attr, c.df.index[missing])
['pypsa/components.py']