fix(add): empty string is real data, not a missing field

validateAddParams() treated '' as falsy and rejected it with "Missing
required field 'data'" — so a legitimate empty file's first write always
failed. Only null/undefined data (with no vector either) is genuinely
absent; '' is real content. Fixed the check, plus the identical bug in
validateUpdateParams() (truncating a file to empty via overwrite hit the
same falsy check) and in update()/transact()'s update planner, where a
plain `Boolean(params.data)`/truthy check on the resolved vector would have
silently skipped both the deferred-embed marker and the eager re-embed for
an emptied value — a stale vector with no path to ever correct itself.

Verified end-to-end: vfs.writeFile('/empty.txt', '') now succeeds,
readFile() returns '', the file lists, and stat() reports size 0; the
existing "should reject empty string as data" tests (unit + integration)
asserted the old buggy behavior and are updated to assert the fixed
contract instead.

tests/integration/brainy-core.integration.test.ts

@@ -337,12 +337,18 @@ describe('Brainy 3.0 Core (Integration Tests - Real AI)', () => {
 
   describe('Error Handling and Edge Cases', () => {
     it('should handle invalid inputs gracefully', async () => {
-      // Empty data is rejected with a clear validation error (8.0 requires a
-      // non-empty `data` or a `vector` — empty string carries no signal to embed).
+      // Empty string is REAL content (e.g. an empty file's first write), not
+      // a missing field — only null/undefined data (with no vector either)
+      // is rejected. See src/utils/paramValidation.ts validateAddParams().
       await expect(brain.add({
         data: '',
         type: 'document'
-      })).rejects.toThrow(/data/)
+      })).resolves.toBeDefined()
+
+      // Missing BOTH data and vector is still the real "nothing to embed" error.
+      await expect(brain.add({
+        type: 'document'
+      } as any)).rejects.toThrow(/data/)
 
       // Test with very long text — valid input, resolves to an id.
       const longText = 'Lorem ipsum '.repeat(10000)