top-level plot collections: fail on empty dict (#10233)

* top-level plot collections: skip empty dict

* tighten schema for plots; require key name

dvc/repo/plots/__init__.py

@@ -487,6 +487,7 @@ def _collect_pipeline_files(repo, targets: List[str], props, onerror=None):
             if isinstance(elem, str):
                 dvcfile_defs_dict[elem] = None
             else:
+                assert elem
                 k, v = next(iter(elem.items()))
                 dvcfile_defs_dict[k] = v
 

dvc/schema.py

@@ -112,7 +112,7 @@ PLOT_DEFINITION = {
     Output.PARAM_PLOT_TITLE: str,
     Output.PARAM_PLOT_TEMPLATE: str,
 }
-SINGLE_PLOT_SCHEMA = {str: vol.Any(PLOT_DEFINITION, None)}
+SINGLE_PLOT_SCHEMA = {vol.Required(str): vol.Any(PLOT_DEFINITION, None)}
 ARTIFACTS = "artifacts"
 SINGLE_ARTIFACT_SCHEMA = vol.Schema({str: ARTIFACT_SCHEMA})
 FOREACH_IN = {