Support sparse sample stats

[michaelosthege]

Some sample stats, such as detailed NUTS tuning or leapfrog information is too big to store for every draw.

By supporting None elements in the sampler_stat group, we could allow samplers to emit this information only sometimes.

For example:

• The mass matrix every time it was changed during tuning.
• Detailed information about divergent samples (see Collect sampler warnings only through stats #6192).