Fix Data.to_heterogeneous() with empty edge_index

[rusty1s]

Also fixes #6299.

[codecov]

Codecov Report

Merging #6304 (78f5592) into master (56cf20f) will increase coverage by 0.00%.
The diff coverage is 77.77%.

@@           Coverage Diff           @@
##           master    #6304   +/-   ##
=======================================
  Coverage   84.56%   84.56%           
=======================================
  Files         380      380           
  Lines       21161    21162    +1     
=======================================
+ Hits        17894    17896    +2     
+ Misses       3267     3266    -1     

Impacted Files	Coverage Δ
torch_geometric/data/hetero_data.py	94.58% <50.00%> (-0.21%)	⬇️
torch_geometric/data/data.py	93.01% <80.00%> (+0.41%)	⬆️
...geometric/explain/algorithm/attention_explainer.py	85.96% <100.00%> (+0.25%)	⬆️
torch_geometric/explain/algorithm/gnn_explainer.py	96.64% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more