[storageimportexport] "url" should not be empty in "externalDocs"

"url" is empty in externalDocs as below in storageimportexport swagger, which does not follow the swagger standard.
```
"externalDocs": {
"url": ""
},
```
For example
https://github.com/Azure/azure-rest-api-specs/blob/master/specification/storageimportexport/resource-manager/Microsoft.ImportExport/stable/2020-08-01/storageimportexport.json#L92
https://github.com/Azure/azure-rest-api-specs/blob/master/specification/storageimportexport/resource-manager/Microsoft.ImportExport/stable/2016-11-01/storageimportexport.json#L138

If load the swagger in editor.swagger.io, you will find it will report the error. See the attachment for details

Open the two linked storageimportexport JSON specifications and inspect the externalDocs objects around the referenced lines. Check the Swagger specification requirement and validate the updated documents in editor.swagger.io; done means the externalDocs url is no longer empty and the validation error is resolved.

Written by the indexing model from the issue text.