fix: don't error if node data file is empty

In PR #728, extra node data validation was introduced. In particular, files without information for either `nodes` or `branches` caused erroring.

This is problematic for test scripts that may produce empty node data in test cases.

This PR removes the eager validation. In the future we could reintroduce it as a warning.
And possibly an error but with opt-out.

Author: corneliusroemer

augur/util_support/node_data_file.py

@@ -1,12 +1,10 @@
 import json
 
-from augur.__version__ import __version__
-from augur.__version__ import is_augur_version_compatible
+from augur.__version__ import __version__, is_augur_version_compatible
 from augur.errors import AugurError
 from augur.io.print import print_err
 from augur.types import ValidationMode
-from augur.validate import validate_json, ValidateError, load_json_schema
-
+from augur.validate import ValidateError, load_json_schema, validate_json
 
 FILTERED_ATTRS = ["generated_by"]
 
@@ -82,11 +80,6 @@ def validate(self):
             raise AugurError(
                 f"`branches` value in {self.fname} is not a dictionary. Please check the formatting of this JSON!"            )
 
-        if not self.nodes and not self.branches:
-            raise AugurError(
-                f"{self.fname} did not contain either `nodes` or `branches`. Please check the formatting of this JSON!"
-        )
-
         if self.validation_mode is not ValidationMode.SKIP and self.is_generated_by_incompatible_augur:
             msg = (
                 f"Augur version incompatibility detected: the JSON {self.fname} was generated by "