Fix modifying plot (#3926)

- no error when modifying plot with no props yet
- do not turn into non-plot if all props are unset

dvc/repo/plots/__init__.py

@@ -18,13 +18,17 @@ class Plots:
         (out,) = self.repo.find_outs_by_path(path)
 
         # This out will become a plot unless it is one already
-        if not out.plot:
+        if not isinstance(out.plot, dict):
             out.plot = {}
 
         for field in unset or ():
             out.plot.pop(field, None)
         out.plot.update(props or {})
 
+        # Empty dict will move it to non-plots
+        if not out.plot:
+            out.plot = True
+
         out.verify_metric()
 
         dvcfile = Dvcfile(self.repo, out.stage.path)