Schemas: fix handling of nullable fields

[hariso]

Description

Data unmarshaling is done in two phases: unmarshaling by Avro, which sometimes represents values with maps, and our own unmarshaling, where we substitute those maps with actual values. For example, "my_field": map[string]any{"int": 1} needs to be replaced with "my_fied": 1.

Value substitution needs to know what's the "parent" of a field, so that it can replace the value for it. Due to a bug, the value substitution for fields that are nullable (i.e. that have null in their union type) was triggered for all fields in the parent object. Because value substitution assumes that the values are in a special format (a map with just one key, where the key is the type, and the value is the actual value), we got an error, because the other fields aren't in that format.

The actually fix is this part, where we check the type of the field. The rest of the changes are mostly comments and a bit of extracting code into methods.

Quick checks:

• I have followed the Code Guidelines.
• There is no other pull request for the same update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.

[maha-hajja]

lgtm! thank you for the fix

[lovromazgon]

Good work 👍