Fix inconsistent null handling in DataFrame Arithmetics (#6770)

* Fix inconsistent null handling in DataFrame Arithmetics

* Fix Null Count and division by zero issues

* Minor changes to restart build and rerun flaky tests

Author: asmirnov82

src/Microsoft.Data.Analysis/PrimitiveColumnContainerHelpers.cs

@@ -11,16 +11,14 @@ internal static class PrimitiveColumnContainerHelpers
         internal static DataFrameBuffer<T> GetOrCreateMutable<T>(this IList<ReadOnlyDataFrameBuffer<T>> bufferList, int index)
             where T : unmanaged
         {
-            ReadOnlyDataFrameBuffer<T> sourceBuffer = bufferList[index];
-            DataFrameBuffer<T> mutableBuffer = sourceBuffer as DataFrameBuffer<T>;
+            var sourceBuffer = bufferList[index];
 
-            if (mutableBuffer == null)
+            if (sourceBuffer is not DataFrameBuffer<T> mutableBuffer)
             {
                 mutableBuffer = DataFrameBuffer<T>.GetMutableBuffer(sourceBuffer);
                 bufferList[index] = mutableBuffer;
             }
 
-
             return mutableBuffer;
         }
     }