In Render tolerate not being passed a context

It is possible for RenderString to be passed to an external renderer if markdown
is set to be rendered by an external renderer. No context is currently sent to these
meaning that this will error out.

Fix go-gitea#16835

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

modules/markup/external/external.go

@@ -14,6 +14,7 @@ import (
 	"runtime"
 	"strings"
 
+	"code.gitea.io/gitea/modules/graceful"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/markup"
 	"code.gitea.io/gitea/modules/process"
@@ -99,7 +100,12 @@ func (p *Renderer) Render(ctx *markup.RenderContext, input io.Reader, output io.
 	}
 
 	if ctx == nil || ctx.Ctx == nil {
-		return fmt.Errorf("RenderContext did not provide context")
+		if ctx == nil {
+			log.Warn("RenderContext not provided defaulting to empty ctx")
+			ctx = &markup.RenderContext{}
+		}
+		log.Warn("RenderContext did not provide context, defaulting to Shutdown context")
+		ctx.Ctx = graceful.GetManager().ShutdownContext()
 	}
 
 	processCtx, cancel := context.WithCancel(ctx.Ctx)