[Bug] Line/area series step on data with nulls produces unexpected results

[alxnddr]

Version

5.5.0

Link to Minimal Reproduction

https://echarts.apache.org/examples/en/editor.html?c=line-simple&code=PYBwLglsB2AEC8sDeAoWsAeBBDEDOAXMmurGAJ4gCmRA5AMYCGYVA5sAE7m0A0J6AE2aMiAbVoBZGL1i0AKgFcqM2gHUqAlXIAWClQDEOEFQGVmphdFoBdEgF8-6cjnxFUpMpRqyAbowA2SrT2jrB4VEZUhLCi_MQegsJi0Ar-_jywAKwADBkAbLmwKWkZAOyFAByFxf7WoR4U1HT-ENDK9aSMHFSMJhT-3kiwoIz0EBRE2QB0AEywDnHoeCwgdFTQmh3o9DBt9GAAcqn-0WAcSnF2JLZ2ANxAA

Steps to Reproduce

1. Have a line/area cartesian chart where first and last data points are null (example)
2. Set areaStyle: {}
3. Set step: end and connectNulls: true props
4. The chart does not appear correctly

Current Behavior

Area polygon is incorrect, the bottom of it is shifted:

Expected Behavior

The area polygon should be correct

Environment

- OS: macos
- Browser: chrome
- Framework: n/a

Any additional comments?

There is a related problem when connectNulls is false:

[Ovilia]

The polygon one is a bug. But the one in Any additional comments seems to be correct.

This looks as expected to me. Step can be one of 'start', 'middle', 'end'.