Merge branch 'PHP-7.4'

* PHP-7.4:
  Handle empty password fast path in caching_sha2_password
  Handle error response during caching_sha2_password auth
  Add support for caching_sha2_password in change user authentication
  Fix unix socket check during caching_sha2_password
  Support auth switch request during caching sha2 auth

diff --cc ext/mysqlnd/mysqlnd_auth.c
Simple merge

diff --cc ext/mysqlnd/mysqlnd_auth.h
Simple merge

diff --cc ext/mysqlnd/mysqlnd_connection.c
Simple merge

diff --cc ext/mysqlnd/mysqlnd_structs.h
Simple merge

diff --cc ext/mysqlnd/mysqlnd_wireprotocol.c
Simple merge

diff --cc ext/mysqlnd/mysqlnd_wireprotocol.h
Simple merge