fix: solve modDatetime type errors

Update modDatetime props type to have null type for components that have modDatetime as props.

src/layouts/Layout.astro

@@ -12,7 +12,7 @@ export interface Props {
   ogImage?: string;
   canonicalURL?: string;
   pubDatetime?: Date;
-  modDatetime?: Date;
+  modDatetime?: Date | null;
   scrollSmooth?: boolean;
 }