Revert "Remove getNullCell"

This reverts commit 0b6a5b7630.

src/public/Terminal.ts

@@ -204,6 +204,7 @@ class BufferApiView implements IBufferApi {
     }
     return new BufferLineApiView(line);
   }
+  public getNullCell(): IBufferCellApi { return new CellData(); }
 }
 
 class BufferLineApiView implements IBufferLineApi {