[AIRFLOW-3887] Downgrade dagre-d3 to 0.4.18 (#4713)

dagre-d3 v0.6.3 has a bug that causes this Javascript error when loading
the Graph View:

    TypeError: previousPaths.merge is not a function

The bug fix [1] has been merged to master, but hasn't been released to
npm yet. This change temporarily downgrades our version of dagre-d3
until dagre-d3 v0.6.4 is released [2]

I also fixed a bug I encountered in the `compile_assets.sh` where the
script would fail if the directory `airflow/www/static/dist/` exists but
is empty.

[1] dagrejs/dagre-d3#350
[2] https://github.com/dagrejs/dagre-d3/blob/5450627790ff42012ef50cef6b0e220199ae4fbe/package.json#L3

GitOrigin-RevId: 72fe43b875bb8458b298c857752d14e1bd09b60e

airflow/www/compile_assets.sh

@@ -19,7 +19,7 @@ set -e
 
 # first bump up package.json manually, commit and tag
 if [ -d airflow/www/static/dist ]; then
-  rm airflow/www/static/dist/*
+  rm -f airflow/www/static/dist/*
 fi
 
 cd airflow/www/