NullPointerException thrown when native query returns 1 row, 1 column and the value is null

[LucaFornelli]

Description

In one of our repository classes, the query expects a String as result type, but it might occasionally return null value.
When this happens a NullPointerException is thrown instead of the expected empty Optional.

Steps to Reproduce

with this repository:

package default

import io.micronaut.data.annotation.Query
import io.micronaut.data.jdbc.annotation.JdbcRepository
import io.micronaut.data.model.query.builder.sql.Dialect
import io.micronaut.data.repository.GenericRepository

@JdbcRepository(dialect = Dialect.MYSQL)
abstract class NullTestRepository implements GenericRepository<String, Long> {

    @Query("select null")
    abstract Optional<String> somethingThatMightSometimesReturnNull()
}

When calling somethingThatMightSometimesReturnNull() a null pointer exception is thrown here:
https://github.com/micronaut-projects/micronaut-data/blob/master/data-runtime/src/main/java/io/micronaut/data/runtime/mapper/ResultReader.java#L50

Since the value is null, calling the .getClass() method will thrown a NullPointerException

Expected Behaviour

When a null value is the result of a query, an empty Optional should be returned.

Actual Behaviour

When a null value is the result of a query, a NullPointerException is thrown.

Environment Information

• Operating System: MacOS 10.15.7
• Micronaut Version: 1.3.6
• Micronaut Data Version: 1.1.0 (also happening in version 1.1.3)
• JDK Version: 1.8

[graemerocher]

PR submitted for this #767