DataSouceBuilder can fail with a NPE when the driver is null

[chanbinme]

This PR fixes a potential NullPointerException in SimpleDataSourceProperties when mapping the driver class name property.

When a SimpleDriverDataSource is created without explicitly setting a driver, calling dataSource.getDriver().getClass() throws NPE because getDriver() returns null.

Added null check to safely handle the case when no driver is configured, returning null instead of throwing NPE.

[wilkinsona]

Thanks for the PR, @chanbinme. Could you please add a test that fails due to an NPE without the proposed change? If I've understood the problem correctly, I think that will require deriving a DataSourceBuilder from an existing SimpleDriverDataSource instance.

[chanbinme]

@wilkinsona

Thank you for the feedback! You're absolutely right about adding a test to demonstrate the NPE issue.

I'll add a test case following the existing naming convention, something like:
buildWhenDerivedFromSimpleDriverDataSourceWithDriverNotSetSucceeds()

While writing the test, I discovered an additional NPE issue in the convertToString method where null values were being passed, causing NullPointerException when trying to call .getName() on a null Class object. I've addressed this as well by adding proper null checks.

[snicoll]

Thanks @chanbinme

[chanbinme]

@snicoll
Thank you for the guidance and for merging this PR! It was a great learning experience working on this issue. Looking forward to contributing more to Spring Boot in the future. 😊