DataSourceBuilder triggers an NPE in Hikari when trying to build a DataSource with a JDBC URL for an unknown driver

[wilkinsona]

DataSourceBuilder.create()
	.url("jdbc:example://localhost:1234/example")
	.type(HikariDataSource.class)
	.build();

java.lang.NullPointerException
	at java.base/java.util.concurrent.ConcurrentHashMap.putVal(ConcurrentHashMap.java:1011)
	at java.base/java.util.concurrent.ConcurrentHashMap.putIfAbsent(ConcurrentHashMap.java:1541)
	at java.base/java.lang.ClassLoader.getClassLoadingLock(ClassLoader.java:671)
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:651)
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:639)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:525)
	at com.zaxxer.hikari.HikariConfig.attemptFromContextLoader(HikariConfig.java:973)
	at com.zaxxer.hikari.HikariConfig.setDriverClassName(HikariConfig.java:483)
	at org.springframework.boot.jdbc.DataSourceBuilder$MappedDataSourceProperty.set(DataSourceBuilder.java:480)
	at org.springframework.boot.jdbc.DataSourceBuilder$MappedDataSourceProperties.set(DataSourceBuilder.java:374)
	at org.springframework.boot.jdbc.DataSourceBuilder.build(DataSourceBuilder.java:193)
	…

It should leave the driver class name unset if it cannot be derived from the JDBC URL.