JDBC: Prevent ContainerDatabaseDriver from throwing NPEs when on classpath but not initialized

[mrotteveel]

When ContainerDatabaseDriver hasn't been used yet to connect, the delegate is null. This causes various JDBC metadata methods of the driver to fail with a NullPointerException. The change prevents this by providing reasonable defaults.

As an example, when org.testcontainers:jdbc is on the classpath, but hasn't been used yet, the following produces a NullPointerException:

DriverManager.drivers()
    .forEach(driver -> System.out.printf("Driver: '%s', version: %d.%d%n", 
        driver.getClass().getName(), driver.getMajorVersion(), driver.getMinorVersion()));

Exception in thread "main" java.lang.NullPointerException
	at org.testcontainers.jdbc.ContainerDatabaseDriver.getMajorVersion(ContainerDatabaseDriver.java:237)
	at nl.lawinegevaar.pg.DriverVersion.lambda$main$1(DriverVersion.java:39)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1655)
	at java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:658)
	at nl.lawinegevaar.pg.DriverVersion.main(DriverVersion.java:39)

The interrogation of the java.sql.Driver metadata shouldn't produce a NPE.

[rnorth]

Thanks @mrotteveel, this sounds like a good and useful improvement. I'm happy to approve.