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/unit/brainy/add.test.ts

@@ -335,15 +335,24 @@ describe('Brainy.add()', () => {
   })
   
   describe('edge cases', () => {
-    it('should reject empty string as data', async () => {
-      // Arrange
+    it('should accept an empty string as real (empty) data', async () => {
+      // Arrange — '' is legitimate content (e.g. an empty file's first
+      // write), not a missing field. Only null/undefined data (with no
+      // vector either) is "missing" — see the separate
+      // 'data and vector are both missing' test above.
       const params = createAddParams({
         data: '',
         type: 'thing'
       })
-      
-      // Act & Assert - Empty string is not valid data
-      await expect(brain.add(params)).rejects.toThrow('Invalid add() parameters: Missing required field \'data\'')
+
+      // Act
+      const id = await brain.add(params)
+
+      // Assert — stored and readable back as empty, not rejected
+      expect(id).toBeDefined()
+      const entity = await brain.get(id)
+      expect(entity).not.toBeNull()
+      expect(entity!.data).toBe('')
     })
     
     it('should handle very long text content', async () => {