The `default` tag in a struct field doesn't support an empty string

[milanvthakor]

Describe the bug
When attempting to set the default value of a string type field to an empty string using the tag default:"", the swag command ignores this property during Swagger specification generation. This occurs even though an empty string is a valid default value according to the Swagger specification documentation.

To Reproduce
Steps to reproduce the behavior:

1. Here is the sample code having the status field with an empty string as a default value.

package main

import (
	"fmt"
	docs "gin-swagger-example/docs"
	"net/http"

	"github.com/gin-gonic/gin"
	swaggerfiles "github.com/swaggo/files"
	ginSwagger "github.com/swaggo/gin-swagger"
)

// Pet example
type Pet struct {
	ID        int      `json:"id"`
	Name      string   `json:"name"`
	PhotoUrls []string `json:"photoUrls"`
	Status    string   `json:"status" default:""`
}

// @BasePath /api/v1

// SavePet example
//
//	@Summary		Save a new pet to the store
//	@Description	Save a new pet to the store
//	@ID				save-pet
//	@Accept			json
//	@Produce		json
//	@Param			pet	body		Pet			true	"Pet"
//	@Success		200		{string}	string			"ok"
//	@Failure		400		{object}	gin.H	"We need ID!!"
//	@Router			/example/save-pet [post]
func SavePet(c *gin.Context) {
	var pet Pet
	if err := c.ShouldBindJSON(&pet); err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
		return
	}

	fmt.Println("Request Pet", pet)
	c.JSON(http.StatusOK, gin.H{"message": "Pet saved successfully"})
}

func main() {
	r := gin.Default()
	docs.SwaggerInfo.BasePath = "/api/v1"
	v1 := r.Group("/api/v1")
	{
		v1.POST("/save-pet", SavePet)
	}
	r.GET("/swagger/*any", ginSwagger.WrapHandler(swaggerfiles.Handler))
	r.Run(":8080")

}

2. Generate the swagger specification using the command: swag init --pd.
3. Check the generated swagger YAML or JSON files. Here is the output for the above code.

basePath: /api/v1
definitions:
  gin.H:
    additionalProperties: {}
    type: object
  main.Pet:
    properties:
      id:
        type: integer
      name:
        type: string
      photoUrls:
        items:
          type: string
        type: array
      status:
        type: string
    type: object
info:
  contact: {}
paths:
  /example/save-pet:
    post:
      consumes:
      - application/json
      description: Save a new pet to the store
      operationId: save-pet
      parameters:
      - description: Pet
        in: body
        name: pet
        required: true
        schema:
          $ref: '#/definitions/main.Pet'
      produces:
      - application/json
      responses:
        "200":
          description: ok
          schema:
            type: string
        "400":
          description: We need ID!!
          schema:
            $ref: '#/definitions/gin.H'
      summary: Save a new pet to the store
swagger: "2.0"

As can be seen above, the default value for the status property in the main.Pet definitions doesn't have a default tag set.

Expected behavior
The tool should handle the empty string as a default value and set it in the generated swagger specification doc. Here is the valid generated doc.

basePath: /api/v1
definitions:
  gin.H:
    additionalProperties: {}
    type: object
  main.Pet:
    properties:
      id:
        type: integer
      name:
        type: string
      photoUrls:
        items:
          type: string
        type: array
      status:
        default: ""
        type: string
    type: object
info:
  contact: {}
paths:
  /example/save-pet:
    post:
      consumes:
      - application/json
      description: Save a new pet to the store
      operationId: save-pet
      parameters:
      - description: Pet
        in: body
        name: pet
        required: true
        schema:
          $ref: '#/definitions/main.Pet'
      produces:
      - application/json
      responses:
        "200":
          description: ok
          schema:
            type: string
        "400":
          description: We need ID!!
          schema:
            $ref: '#/definitions/gin.H'
      summary: Save a new pet to the store
swagger: "2.0"

Your swag version
e.g. 1.16.6

Your go version
e.g. 1.24.0

Desktop (please complete the following information):

• OS: Ubuntu 24.4.2 LTS
• Browser: Brave
• Version: 1.80.125

[milanvthakor]

I have fixed the issue and created the PR for the same. Please review it. Thanks!