Allow captionless media posts while rejecting truly empty posts

Hop-State: A_06FPA2RFGPJ3QTNYV64F90R
Hop-Proposal: R_06FPA2PG7XQ9EJHPV0AMJHG
Hop-Task: T_06FPA1X78BHAFBEY6GBDNT0
Hop-Attempt: AT_06FPA1X78BRF2NXD1SDAYC0

src/lib/posts/content-policy.ts

@@ -0,0 +1,6 @@
+export function hasPublishablePostContent(
+    content: string,
+    mediaIds: readonly string[] | undefined,
+): boolean {
+    return content.trim().length > 0 || (mediaIds?.length ?? 0) > 0;
+}