fix: ensure API consistency for /tasks and empty array responses (#1988)

- Renames the `/tasks/all` endpoint to `/tasks` for consistency with
other collection endpoints like `/projects` and `/labels`
- Returns `[]` instead of `null` for empty pagination results across all
list endpoints
- Updates the frontend service to use the new endpoint path
- Updates API token tests to use the new endpoint path

Fixes #1984

pkg/web/handler/read_all.go

@@ -21,6 +21,7 @@ import (
 	"fmt"
 	"math"
 	"net/http"
+	"reflect"
 	"strconv"
 
 	vconfig "code.vikunja.io/api/pkg/config"
@@ -128,6 +129,13 @@ func (c *WebHandler) ReadAllWeb(ctx echo.Context) error {
 		return HandleHTTPError(err)
 	}
 
+	// Ensure we return an empty array instead of null when there are no results.
+	// We need to use reflection here because a nil slice wrapped in an interface{}
+	// is not equal to nil (the interface contains a nil value but is not nil itself).
+	if result == nil || (reflect.ValueOf(result).Kind() == reflect.Slice && reflect.ValueOf(result).IsNil()) {
+		result = []interface{}{}
+	}
+
 	err = ctx.JSON(http.StatusOK, result)
 	if err != nil {
 		return HandleHTTPError(err)