Inconsistency between `ArcticTransaction` and `Arctic` writes of `None`

[tjw45]

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Arctic Version

# All

Arctic Store

# VersionStore

Platform and version

All

Description of problem and/or code sample that reproduces the issue

There is an inconsistency between how plain Arctic writes and ArcticTransaction writes behave when given None; Arctic will write the symbol, whereas ArcticTransaction will not. Additionally, the copy script doesn't play nicely when trying splice empty DataFrames (or None) onto existing data.

[shashank88]

@tjw45 let's try to get this merged and close this. Think the build is still having some issues, taking a look