feat: improve error handling of column.stability when given a colum…

…n that contains only None (#388)

Closes #319 

### Summary of Changes

The docstring of the method clarifies that the stability is not defined
if a column contains only null values.
The method raises a ValueError if the column contains only null values.

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---------

Co-authored-by: patrikguempel <[email protected]>
Co-authored-by: Simon Breuer <[email protected]>

src/safeds/data/tabular/containers/_column.py

@@ -504,7 +504,7 @@ def stability(self) -> float:
         \frac{\text{number of occurrences of most common non-null value}}{\text{number of non-null values}}
         $$
 
-        The stability cannot be calculated for a column with only null values.
+        The stability is not definded for a column with only null values.
 
         Returns
         -------
@@ -520,7 +520,7 @@ def stability(self) -> float:
             raise ColumnSizeError("> 0", "0")
 
         if self.all(lambda x: x is None):
-            raise ValueError("Stability cannot be calculated for a column with only null values.")
+            raise ValueError("Stability is not definded for a column with only null values.")
 
         return self._data.value_counts()[self.mode()[0]] / self._data.count()
 

tests/safeds/data/tabular/containers/_column/test_stability.py

@@ -33,5 +33,5 @@ def test_should_raise_column_size_error_if_column_is_empty() -> None:
 
 def test_should_raise_value_error_if_column_contains_only_none() -> None:
     column: Column[Any] = Column("A", [None, None])
-    with pytest.raises(ValueError, match="Stability cannot be calculated for a column with only null values."):
+    with pytest.raises(ValueError, match="Stability is not definded for a column with only null values."):
         column.stability()