Allow API to create file on empty repo (#19224)

This PR adds the necessary work to make it possible to create files on empty
repos using the API.

Fix #10993

Signed-off-by: Andrew Thornton <art27@cantab.net>

routers/api/v1/repo/file.go

@@ -233,9 +233,6 @@ func CreateFile(ctx *context.APIContext) {
 	//     "$ref": "#/responses/error"
 
 	apiOpts := web.GetForm(ctx).(*api.CreateFileOptions)
-	if ctx.Repo.Repository.IsEmpty {
-		ctx.Error(http.StatusUnprocessableEntity, "RepoIsEmpty", fmt.Errorf("repo is empty"))
-	}
 
 	if apiOpts.BranchName == "" {
 		apiOpts.BranchName = ctx.Repo.Repository.DefaultBranch