Fix: reject empty/space-only content in update_chunk API (#14082)

Closes #6541

### What problem does this PR solve?

Add content validation to `update_chunk` (SDK and non-SDK) to reject
empty or whitespace-only content before it reaches the embedding model.

**Before:** Calling `update_chunk` with space-only content (like `" "`,
`""`, `"\n"`) bypassed validation and was sent directly to the embedding
model, which returned an error. This was the same bug previously fixed
for `add_chunk` in #6390, but `update_chunk` was missed.

**After:** Empty/whitespace-only content is caught by validation and
returns an error: `` `content` is required ``

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

common/string_utils.py

@@ -71,3 +71,7 @@ def clean_markdown_block(text):
 
     # Return text with surrounding whitespace removed
     return text.strip()
+
+
+def is_content_empty(content: str) -> bool:
+    return content is None or not str(content).strip()