Fix fetch sub-query aggregation null pointer

[flyingsilverfin]

Usage and product changes

We fix a bug where a Fetch query with a Get-Aggregate subquery that returned an empty (ie. undefined) answer throw a null pointer exception.

For example this used to error if 'Alice' doesn't have any salaries in the database, since a 'sum' is undefined for 0 entries.

match
$x isa person, has name $n; $n == "Alice";
fetch
$n as "name";
total-salary: {
  match $x has salary $s;
  get $s; 
  sum $s;
};

We now return the following JSON structure

[
  {
    "name": {"value": "Alice",  "type":  {"label": "name", "root": "attribute", "value_type": "string"}},
    "total-salary": null
  }
]

Implementation

• We allow 'null' in our internal Fetch answer response data structure
• We package 'null' as an empty 'ReadableConcept' in the protobuf answer message
• We update typedb-behaviour to a new version that includes the relevant test

[vaticle-bot]

PR Review Checklist

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Trivial Change

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