Unnecessary warning fills up the webapp logging file, this happens every time a db pool creates a new connection and no classname is given.
>> Not loading a JDBC driver as driverClassName property is null.

Using the same webapp for mysqljar and mariadbjar system may use a configuration without driverClassName field. Destination tomcat may provide both or one of jdbc jar files without a webapp developer knowledge.

What is an empty driverClassName="" attribute was given then it's an explicit choice and Tomcat did not need to write a warning log?

See source code:
https://github.com/apache/tomcat/blob/ed8cc4f3beddbdc7d9634cc287660322804007fd/modules/jdbc-pool/src/main/java/org/apache/tomcat/jdbc/pool/PooledConnection.java#L292

if (poolProperties.getDriverClassName()==null) {
  //rely on DriverManager
  log.warn("Not loading a JDBC driver as driverClassName property is null.");
} else {...

See contextxml db connection example:
<!-- use mariajdbc if found then mysqljdbc -->
<Resource name="jdbc/mydb" auth="Container" type="javax.sql.DataSource"  factory="org.apache.tomcat.jdbc.pool.DataSourceFactory"
  initialSize="2" maxActive="300" maxIdle="10" maxWait="30000"
  username="myuser" password="mypwd"
  url="jdbc:mysql://127.0.0.1:3306/mydb?permitMysqlScheme&amp;useUnicode=true&amp;characterEncoding=utf8"
  validationQuery="SELECT 1" validationInterval="6000" validationQueryTimeout="10"
  rollbackOnReturn="true" removeAbandoned="true" removeAbandonedTimeout="14400" logAbandoned="false"
  testOnBorrow="true" maxAge="300000" timeBetweenEvictionRunsMillis="60000"
  defaultTransactionIsolation="READ_COMMITTED"
/>