NPEs thrown by serialization for mandatory fields could have better error messages

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When serializing a record with mandatory fields, there is no null check. For a mandatory string field, fast-avro generates code like this:

```
if (((CharSequence) data.get(1)) instanceof Utf8) {
(encoder).writeString(((Utf8)((CharSequence) data.get(1))));
} else {
(encoder).writeString(((CharSequence) data.get(1)).toString());
}
```

Line 4 will throw a NPE due to calling `toString()` on the null field.

Throwing a NPE is not necessarily a problem in and of itself (and we likely need to preserve that behavior in order to be compatible with vanilla Avro) but a better error message would be welcome. For example, the generated code might instead look like:

```
Object value = data.get(1);
if (value instanceof Utf8) {
(encoder).writeString((Utf8) value);
} else if (value == null) {
throw new NullPointerException("Field fieldName is not allowed to be null");
} else {
(encoder).writeString(value.toString());
}
```

This adds one more check on the hot path, though in the end the VM must do this check anyway when dereferencing, so it might not matter in terms of bytecode/JIT.

The issue names no file or test; start by searching the serializer or code-generation path for mandatory string fields and the shown writeString branch. Check the existing serialization tests, then verify that null mandatory values still fail with a clearer field-specific NullPointerException without changing non-null output or nullability behavior.

Written by the indexing model from the issue text.