[java-bigquerystorage] ReadSession: selectedFields on null record coalesced to 0 value

#### Environment details

OS type and version: Linux, Ubuntu 22.04
Java version: 11
version(s): 2.41.1, 2.44.0

#### Steps to reproduce

Considering this table

With the following data

Reading the table with `AVRO` format without any selected fields gives the expected result:
```json
{"required": {"int": 0, "string": "0"}, "optional": null, "repeated": []}
{"required": {"int": 1, "string": "1"}, "optional": {"int": 1, "string": "1"}, "repeated": [{"int": 1, "string": "1"}]}
{"required": {"int": 2, "string": "2"}, "optional": {"int": 2, "string": "2"}, "repeated": [{"int": 2, "string": "2"}]}
{"required": {"int": 3, "string": "3"}, "optional": {"int": 3, "string": "3"}, "repeated": [{"int": 3, "string": "3"}]}
{"required": {"int": 4, "string": "4"}, "optional": {"int": 4, "string": "4"}, "repeated": [{"int": 4, "string": "4"}]}
{"required": {"int": 5, "string": "5"}, "optional": {"int": 5, "string": "5"}, "repeated": [{"int": 5, "string": "5"}]}
{"required": {"int": 6, "string": "6"}, "optional": {"int": 6, "string": "6"}, "repeated": [{"int": 6, "string": "6"}]}
{"required": {"int": 7, "string": "7"}, "optional": {"int": 7, "string": "7"}, "repeated": [{"int": 7, "string": "7"}]}
{"required": {"int": 8, "string": "8"}, "optional": {"int": 8, "string": "8"}, "repeated": [{"int": 8, "string": "8"}]}
{"required": {"int": 9, "string": "9"}, "optional": {"int": 9, "string": "9"}, "repeated": [{"int": 9, "string": "9"}]}
```

When now setting the selected fields to `["required", "optional.int"]`, the library returns the following
```json
{"required": {"int": 0, "string": "0"}, "optional": {"int": 0}}
{"required": {"int": 1, "string": "1"}, "optional": {"int": 1}}
{"required": {"int": 2, "string": "2"}, "optional": {"int": 2}}
{"required": {"int": 3, "string": "3"}, "optional": {"int": 3}}
{"required": {"int": 4, "string": "4"}, "optional": {"int": 4}}
{"required": {"int": 5, "string": "5"}, "optional": {"int": 5}}
{"required": {"int": 6, "string": "6"}, "optional": {"int": 6}}
{"required": {"int": 7, "string": "7"}, "optional": {"int": 7}}
{"required": {"int": 8, "string": "8"}, "optional": {"int": 8}}
{"required": {"int": 9, "string": "9"}, "optional": {"int": 9}}
```

The 1st record is expected to return `{"required": {"int": 0, "string": "0"}, "optional": null}` instead. Somehow its value gets coalesced into 0.