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/vfs/vfs.unit.test.ts

@@ -53,6 +53,35 @@ describe('VirtualFileSystem - Production Tests', () => {
       expect(exists).toBe(true)
     })
 
+    it('should write and read an empty (0-byte) file end-to-end', async () => {
+      // Pin: validateAddParams() used to treat '' as a missing 'data' field
+      // (falsy check), so a legitimate empty file's FIRST write threw
+      // "Missing required field 'data'". '' is real content, not an absent
+      // field — only null/undefined is absent.
+      const path = '/empty.txt'
+
+      await vfs.writeFile(path, '')
+
+      const result = await vfs.readFile(path)
+      expect(result.toString()).toBe('')
+
+      const exists = await vfs.exists(path)
+      expect(exists).toBe(true)
+
+      const stats = await vfs.stat(path)
+      expect(stats.size).toBe(0)
+      expect(stats.isFile()).toBe(true)
+
+      // The file lists like any other.
+      const entries = await vfs.readdir('/') as string[]
+      expect(entries).toContain('empty.txt')
+
+      // Overwriting it back to empty (truncate) must also succeed.
+      await vfs.writeFile(path, 'not empty anymore')
+      await vfs.writeFile(path, '')
+      expect((await vfs.readFile(path)).toString()).toBe('')
+    })
+
     it('should handle binary files', async () => {
       const binaryData = Buffer.from([0x00, 0x01, 0x02, 0xFF])
       const path = '/binary.dat'