fix: 🐛 embedded struct test failed with custom datatypes

[aclich]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

Refer to #6288
Fix embedded struct with custom datatypes and *time.time will failed the test.

User Case Description

Test should pass.

[aclich]

mssql will have type issue while inserting null value into varbinary(max) type.

/home/diad/Developer/hbs4/gorm/tests/embedded_struct_test.go:130 mssql: Implicit conversion from data type nvarchar to varbinary(max) is not allowed. Use the CONVERT function to run this query.
[16347.552ms] [rows:0] INSERT INTO "hn_posts" ("title","url","upvotes","user_id","user_name","user_email","user_age","user_content","user_content_ptr","user_birthday","user_birthday_ptr") OUTPUT INSERTED."id" VALUES ('embedded_pointer_type','',0,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL);

reference https://stackoverflow.com/questions/11411854/implicit-conversion-from-data-type-nvarchar-to-varbinarymax-is-not-allowed
Probably same issue as denisenkom/go-mssqldb#530, need a driver fix.

[aclich]

commit dc149c1
Modify the custom datatypes to string for preventing mssql driver issue.
Reopen PR, fixing embedded struct should be nil.