Commits on Nov 9, 2016

1. Implement reading from null safe dereferences …

   Null safe dereferences make handling null or missing values shorter.
   Compare without:
   ```
   if (ctx._source.missing != null && ctx._source.missing.foo != null) {
     ctx._source.foo_length = ctx.source.missing.foo.length()
   }
   ```
   
   To with:
   ```
   Integer length = ctx._source.missing?.foo?.length();
   if (length != null) {
     ctx._source.foo_length = length
   }
   ```
   
   Combining this with the as of yet unimplemented elvis operator allows
   for very concise defaults for nulls:
   ```
   ctx._source.foo_length = ctx._source.missing?.foo?.length() ?: 0;
   ```
   
   Since you have to start somewhere, we started with null safe dereferenes.
   
   Anyway, this is a feature borrowed from groovy. Groovy allows writing to
   null values like:
   ```
   def v = null
   v?.field = 'cat'
   ```
   And the writes are simply ignored. Painless doesn't support this at this
   point because it'd be complex to implement and maybe not all that useful.
   
   There is no runtime cost for this feature if it is not used. When it is
   used we implement it fairly efficiently, adding a jump rather than a
   temporary variable.
   
   This should also work fairly well with doc values.

   

   nik9000 committed Nov 9, 2016
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