FIX: if statement nesting + redundant docstring entry

river/covariance/emp.py

@@ -14,7 +14,7 @@ class SymmetricMatrix(abc.ABC):
 
     @property
     @abc.abstractmethod
-    def matrix(self):
+    def matrix(self) -> dict:
         ...
 
     def __getitem__(self, key):
@@ -196,8 +196,6 @@ def _update_from_state(self, n: int, mean: dict, cov: float | dict):
             A dictionary of variable means.
         cov
             A dictionary of covariance or variance values.
-        ddof
-            Degrees of freedom for covariance calculation. Defaults to 1.
 
         Raises
         ----------
@@ -226,10 +224,9 @@ def _update_from_state(self, n: int, mean: dict, cov: float | dict):
             except KeyError:
                 self._cov[i, i] = stats.Var(self.ddof)
             if isinstance(cov, dict):
-                if isinstance(cov, dict):
-                    cov_ = cov[i, i]
-                else:
-                    cov_ = cov
+                cov_ = cov[i, i]
+            else:
+                cov_ = cov
             self._cov[i, i] += stats.Var._from_state(n=n, m=mean[i], sig=cov_, ddof=self.ddof)
 
     @classmethod