fix: skip parent_id when serializing span when it actually doesn't have one

[krynju]

Previously we would serialize API.INVALID_SPAN_CONTEXT here if there was no parent, but that would result in a parent span id equal to 0x00, which is technically a valid id Some systems expect no value when a span is the root span and complain if there is a 0x00 span that can't be found, so this fixes the behavior

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.32%. Comparing base (675aa79) to head (006658f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #101 +/- ## ========================================== + Coverage 74.26% 74.32% +0.05% ========================================== Files 46 46 Lines 1438 1441 +3 ========================================== + Hits 1068 1071 +3 Misses 370 370 ```

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