Fix boolean operators precedence

Created by: masci

This is a fix for a bug I've introduced with #3973

Due to operator precedence, even if addedTimeImport is true, the if branch is executed and it results in duplicated import time statements in the generated code.

Steps to reproduce

Use this definition (notice User has 2 date-time related fields, joined and times):

openapi: 3.0.0

info:
  title: Sample OpenAPI Specification
  version: 0.0.1

servers:
  - url: http://localhost:8080/
    description: Example API Service

components:
  schemas:
    'User':
      type: object
      required:
        - display_name
        - email
      properties:
        name:
          type: string
          readOnly: true
        display_name:
          type: string
          maxLength: 20
          minLength: 1
        email:
          type: string
          format: email
        joined:
          type: string
          format: date-time
        times:
          type: array
          items:
            type: string
            format: date-time
            #        from_date:
            #type: string
            #format: date-time

    'ErrorMessage':
      type: object
      required:
        - error_code
        - error_message
      properties:
        error_code:
          type: string
        error_message:
          type: string

paths:
  /users/{user_id}:
    parameters:
      - name: user_id
        in: path
        description: ID of a user
        required: true
        schema:
          type: string
    get:
      description: Gets a user
      operationId: get_user
      responses:
        '200':
          description: User found
          content:
            'application/json':
              schema:
                $ref: '#/components/schemas/User'
        'default':
          description: Unexpected error
          content:
            'application/json':
              schema:
                $ref: '#/components/schemas/ErrorMessage'

With the latest version (4.1.3), run:

openapi-generator generate -g go -i api.yaml -o go/

Notice the wrong import statement in go/model_user.go:

import (
        "time"
        "time"
)

cc: @antihax @bvwells @grokify @kemokemo @bkabrda

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