Fix SingletonCoder serialization

[RustedBones]

If singleton in SingletonCoder was initialized, the coder could not be deserialized.

java.lang.IllegalArgumentException: unable to deserialize Custom DoFn With Execution Info
...
Cause: java.io.InvalidClassException: com.example.MyObject$; no valid constructor

[codecov]

Codecov Report

Merging #4691 (94a35b2) into main (470c80f) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4691   +/-   ##
=======================================
  Coverage   60.64%   60.65%           
=======================================
  Files         286      286           
  Lines       10453    10453           
  Branches      694      694           
=======================================
+ Hits         6339     6340    +1     
+ Misses       4114     4113    -1     

Impacted Files	Coverage Δ
...src/main/scala/com/spotify/scio/coders/Coder.scala	85.84% <ø> (+0.44%)	⬆️

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[shnapz]

Interesting, good catch on LazyCoder!
But what is the use case? I thought Singleton is used only for scala Object

[RustedBones]

@shnapz The problem comes when users have object in their model.
This can happen with ADTs, eg:

sealed trait Color
case object Red extends Color
case object Green extends Color
case object Blue extends Color

The disjunction coder for Color will delegate to singleton coders.
The previous code was working in most cases as case objects are serializable. However, it does not when:

• the object is not a case object
• the case object extends a class with parameter as explained here

The trick here is to only serialize the supplier in the coder.

In practice, Coder are serialized before being used. So the lazy val won't be initialized, (serialized as null).
But in some test cases, beam serializes the coder after usage, hence the observed failure.