Prevent serializing input for legacy split widgets

[DylanPiercey]

Description

In Marko 3 split components do not have access to their input, however modern split components do. This PR prevents serializing input for legacy split components to increase performance in that case.

This is a subset of f4f89c9

Checklist:

• I have read the CONTRIBUTING document and have signed (or will sign) the CLA.
• I have updated/added documentation affected by my changes.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #1497 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1497      +/-   ##
==========================================
- Coverage   90.51%   90.50%   -0.01%     
==========================================
  Files         352      352              
  Lines       12582    12586       +4     
==========================================
+ Hits        11388    11391       +3     
- Misses       1194     1195       +1     

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2063f2d...4fde672. Read the comment docs.