ghstack: fix stdout check in sapling_land

Summary:
Current behavior:

```
File "/Users/zhaolong/fbsource/fbcode/eden/scm/ghstack/sapling_land.py", line 88, in main
  rebase_base = mappings[s.oid][0]
KeyError: '4cf7c5a37180fd76afac15397ecfa1719fdc0d7b'
```

Reviewed By: bolinfest

Differential Revision: D41140977

fbshipit-source-id: fc2bcd5e5209422b7ab8747eed5b4b4278367a71

Author: zzl0

eden/scm/ghstack/sapling_land.py

@@ -75,14 +75,12 @@ def main(pull_request: str,
         rebase_base = default_branch_oid
         for s in stack:
             stdout = sh.run_sapling_command("rebase", "--keep", "-s", s.oid, "-d", rebase_base, "-q", "-T", "{nodechanges|json}")
-            # If there is no output, it appears that '""' is returned as opposed
-            # to '{}', which is a little weird...
-            if not stdout or stdout == '""':
+            mappings = json.loads(stdout) if stdout else None
+            if not mappings:
                 # If there was no stdout, then s.oid was not rebased because
                 # its parent is already the existing `rebase_base`.
                 rebase_base = s.oid
             else:
-                mappings = json.loads(stdout)
                 rebase_base = mappings[s.oid][0]
 
         # Advance base to head to "close" the PR for all PRs.