AccountService: fix PATCH error-path response handling

Added early-return checks in AccountService PATCH handling to avoid
continuing processing after an error response is already set.

Earlier, when no auth-method fields were present in the request,
the auth patch flow could still contribute success semantics.
In error scenarios, this could produce mixed outcomes in one response
(for example, Success with PropertyNotWritable for MaxPasswordLength)
and could also cause PATCH requests for valid writable properties to
appear to hang or time out after an earlier failure path.

With this change:
- Auth-method processing returns immediately when no auth fields are
  present.
- HTTP 200 is set only when status is still OK.
- Processing stops immediately after validation/helper errors.
- Read-only property updates such as MaxPasswordLength return only
  PropertyNotWritable, as expected.

No Redfish schema or property additions in this commit.
Behavior-only change in AccountService PATCH handling.

Tested:
- EVB2600
  PATCH /redfish/v1/AccountService with {"MaxPasswordLength":30}
  response contains only PropertyNotWritable.
- EVB2600
  PATCH with valid writable properties, for example:
  {"AccountLockoutDuration":301,"AccountLockoutThreshold":32}
  request completes normally without hang/abrupt client close.
- Redfish-Service-Validator: PASS.

Change-Id: Ie6e66c432d38ef29f4aadd70067440d15a93a95f
Signed-off-by: Sandeep Akant Patil <sandeepap@ami.com>