dialectKey为空导致NullPointerException

[Elin-Zhou]

https://github.com/pagehelper/Mybatis-PageHelper/blob/9435e5252e70babd05539c857f0fce17c9c7cb3d/src/main/java/com/github/pagehelper/page/PageAutoDialect.java#L235

当判断dialectKey == null时，执行autoDialectDelegate.extractDialect(dialectKey, ms, dataSource, properties); autoDialectDelegate的默认实现类是DataSourceNegotiationAutoDialect，会执DataSourceNegotiationAutoDialect.java:73代码，urlMap.containsKey(dialectKey)，dialectKey为null，直接抛出NPE