[POST user endpoint] Not enough validations for fields: "lastName", "firstName"

[AlexandraPopovaz]

Steps to reproduce
Scenario 1: if "lastName" is a number
send a POST user request with data:
{ title: "should return an error if lastName is a number", role: "student" as Role, field: "lastName", value: 123, expected: 400, },
Actual behavior 200 code
Expected behavior 400 error code

Scenario 2: if "firstName" is an empty object
send a POST user request with data:
{ title: "should return an error if firstName is an empty object", role: "student" as Role, field: "firstName", value: {}, expected: 400, }
Actual behavior 500 code
Expected behavior 400 error code

Scenario 3: if "lastName" is an empty object
send a POST user request with data:
{ title: "should return an error if lastName is an empty object", role: "student" as Role, field: "lastName", value: {}, expected: 400, }
Actual behavior 500 code
Expected behavior 400 error code

Scenario 4: if "lastName" is an empty Array
send a POST user request with data:
{ title: "should return an error if lastName is an empty Array", role: "student" as Role, field: "lastName", value: [], expected: 400, }
Actual behavior 500 code
Expected behavior 400 error code