bugfix: fix mariadb unable to create XA connection

[funky-eyes]

Ⅰ. Describe what this PR did

如果引入了mariadb的驱动却没有引入mysql的，那么druid的MySqlUtils只会反射mysql的驱动来创建xaconnection，此时会抛出npe，mariadb xa模式就无法正常使用

Ⅱ. Does this pull request fix one issue?

fixes #3696

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-commenter]

Codecov Report

Merging #3695 (b0d78c4) into develop (69d1297) will increase coverage by 0.00%.
The diff coverage is 18.18%.

❗ Current head b0d78c4 differs from pull request most recent head c2f8083. Consider uploading reports for the commit c2f8083 to get more accurate results

@@            Coverage Diff             @@
##             develop    #3695   +/-   ##
==========================================
  Coverage      51.31%   51.32%           
- Complexity      3576     3577    +1     
==========================================
  Files            646      646           
  Lines          21809    21809           
  Branches        2738     2737    -1     
==========================================
+ Hits           11191    11193    +2     
+ Misses          9480     9479    -1     
+ Partials        1138     1137    -1     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/io/seata/rm/datasource/util/XAUtils.java	20.00% <18.18%> (+8.00%)	4.00 <2.00> (+1.00)

[xingfudeshi]

LGTM.

[qianxunclub]

SpringBoot:2.4.1
grpc-spring-boot-starter:2.11.0.RELEASE
mariadb
mybatis-plus

测试事务可以正常使用

[slievrly]

LGTM