Errors on motif with disconnected nodes

Encountered an error when feeding in some motifs that had isolated nodes.

Code:
```python
import networkx as nx
from grandiso import find_motifs

host = nx.fast_gnp_random_graph(3, 0.5, directed=True)

motif = nx.DiGraph()
motif.add_node(0)
motif.add_node(1)
motif.add_node(2)
motif.add_edge(0, 1)
motif.add_edge(1, 0)

grandiso_motifs = find_motifs(motif, host, count_only=False)
```

Error:
```pytb
File ~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:204, in get_next_backbone_candidates(backbone, motif, host, interestingness, next_node, directed, is_node_structural_match, is_node_attr_match, is_edge_attr_match, isomorphisms_only)
[201](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:201) _greatest_backbone_count = motif_backbone_connections_count
[202](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:202) # Now we have _node_with_greatest_backbone_count as the best candidate
[203](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:203) # for `next_node`.
--> [204](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:204) next_node = max(
[205](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:205) _nodes_with_greatest_backbone_count,
[206](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:206) key=lambda node: interestingness.get(node, 0.0),
[207](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:207) )
[209](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:209) # Now we have a node `next_node` which we know is connected to the current
[210](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:210) # backbone. Get all edges between `next_node` and nodes in the backbone,
[211](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:211) # and verify that they exist in the host graph:
[212](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:212) # `required_edges` has the form (prev, self, next), with non-values filled
[213](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:213) # with None. That way we can easily remember and store the roles of the
[214](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:214) # node IDs in the next step.
[215](https://file+.vscode-resource.vscode-cdn.net/Users/ben.pedigo/code/skedits/skedits-app/skedits/~/code/skedits/skedits-app/skedits/.venv/lib/python3.11/site-packages/grandiso/__init__.py:215) required_edges = []

ValueError: max() arg is an empty sequence
```

Totally understand if you want to punt on how whether/how to execute the search with these disconnected motifs, but some documentation and/or early value erroring around this would be helpful. Took me a while to figure out what I was doing wrong (since the NetworkX will execute for these cases).