Properly filter issue list given no assignees filter (#31522)

Quick fix #31520. This issue is related to #31337.

Author: kemzeb

Diff for: modules/indexer/issues/dboptions.go

@@ -44,6 +44,12 @@ func ToSearchOptions(keyword string, opts *issues_model.IssuesOptions) *SearchOp
 		searchOpt.ProjectID = optional.Some[int64](0) // Those issues with no project(projectid==0)
 	}
 
+	if opts.AssigneeID > 0 {
+		searchOpt.AssigneeID = optional.Some(opts.AssigneeID)
+	} else if opts.AssigneeID == -1 { // FIXME: this is inconsistent from other places
+		searchOpt.AssigneeID = optional.Some[int64](0)
+	}
+
 	// See the comment of issues_model.SearchOptions for the reason why we need to convert
 	convertID := func(id int64) optional.Option[int64] {
 		if id > 0 {
@@ -57,7 +63,6 @@ func ToSearchOptions(keyword string, opts *issues_model.IssuesOptions) *SearchOp
 
 	searchOpt.ProjectColumnID = convertID(opts.ProjectColumnID)
 	searchOpt.PosterID = convertID(opts.PosterID)
-	searchOpt.AssigneeID = convertID(opts.AssigneeID)
 	searchOpt.MentionID = convertID(opts.MentionedID)
 	searchOpt.ReviewedID = convertID(opts.ReviewedID)
 	searchOpt.ReviewRequestedID = convertID(opts.ReviewRequestedID)

Diff for: modules/indexer/issues/indexer_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 
 	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/issues"
 	"code.gitea.io/gitea/models/unittest"
 	"code.gitea.io/gitea/modules/indexer/issues/internal"
 	"code.gitea.io/gitea/modules/optional"
@@ -188,6 +189,11 @@ func searchIssueByID(t *testing.T) {
 			},
 			expectedIDs: []int64{6, 1},
 		},
+		{
+			// NOTE: This tests no assignees filtering and also ToSearchOptions() to ensure it will set AssigneeID to 0 when it is passed as -1.
+			opts:        *ToSearchOptions("", &issues.IssuesOptions{AssigneeID: -1}),
+			expectedIDs: []int64{22, 21, 16, 15, 14, 13, 12, 11, 20, 5, 19, 18, 10, 7, 4, 9, 8, 3, 2},
+		},
 		{
 			opts: SearchOptions{
 				MentionID: optional.Some(int64(4)),