resolve the delete issue of mysqluser and psqluser

[buhongw7583c]

Closes #1142

What this PR does / why we need it:
If the username is empty in the spec of mysqluser.yaml and postgresqluser.yaml file, the user could not be successfully deleted from database.

Special notes for your reviewer:

1. Issue with mysqluser:
   Before the change, apply the below yaml file, that an entry with user name 'mysqluser-sample-hong1' will be created .

apiVersion: azure.microsoft.com/v1alpha1
kind: MySQLUser
metadata:
  name: mysqluser-sample-hong1
spec:
  server: mysqlserver-sample3
  dbName: mysqldatabase-sample
  resourceGroup: resourcegroup-azure-operators
  roles: 
  #now only supports to grant privileges to a new user, the privileges could be one or more of the below
  #SELECT, INSERT, UPDATE, DELETE, CREATE, DROP, RELOAD, PROCESS, REFERENCES, INDEX, ALTER, SHOW DATABASES, 
  #CREATE TEMPORARY TABLES, LOCK TABLES, EXECUTE, REPLICATION SLAVE, REPLICATION CLIENT, 
  #CREATE VIEW, SHOW VIEW, CREATE ROUTINE, ALTER ROUTINE, CREATE USER, EVENT, TRIGGER
  # This adds the privileges to the specified database
    - SELECT
  # Specify a specific username for the user
  # username: mysqluser-sample
  # Specify adminSecret and adminSecretKeyVault if you want to 
  # read the MYSQL server admin creds from a specific keyvault secret
  # adminSecret: mysqlserver-sample
  # adminSecretKeyVault: asokeyvault

Use kubectl delete mysqluser, the user instance was deleted from cli but still in the database.
After the change, delete the mysqluser will be successful from both cli and database.

2. Issue with postgresqluser:

Before the change, apply the yaml file without specify the username.

apiVersion: azure.microsoft.com/v1alpha1
kind: PostgreSQLUser
metadata:
  name: psqluser-sample
spec:
  server: postgresqlserver-hongsample1
  dbName: postgresqldatabase-sample
  resourceGroup: resourcegroup-azure-operators
  # The Azure Database for PostgreSQL server is created with the 3 default roles defined.
  # azure_pg_admin
  # azure_superuser
  # your server admin user
  roles:
    - "azure_pg_admin"
   # Specify a specific username for the user
  # username: psqluser-sample
  # Specify adminSecret and adminSecretKeyVault if you want to 
  # read the PSQL server admin creds from a specific keyvault secret
  # adminSecret: postgresqlserver-sample
  # adminSecretKeyVault: asokeyvault

Delete the user instance from cli, but the user still in the database.

After the change, delete operation will delete the user both from cli and database.

How does this PR make you feel:

If applicable:

• this PR contains documentation
• this PR contains tests

[buhongw7583c]

Have tested the code change on AKS cluster using helm chart deployment. The MySQLuser and PostgresqlUser could be added/deleted without issue or error throw out.

[jananivMS]

Looks good. tested the changes and worked well.