Update stack size filtering in app.py to handle missing values

- Added a fillna(1) method to ensure that missing values in the stack size filtering logic are replaced with 1, improving data integrity during filtering operations.

app.py

@@ -260,6 +260,7 @@ with tab2:
                     if stack_parse_var == 'Specific' and stack_names:
                         working_df = working_df[working_df['stack'].isin(stack_names)]
                     if low_stack_size_var and high_stack_size_var:
+                        working_df['stack_size'] = working_df['stack_size'].fillna(1)
                         working_df = working_df[working_df['stack_size'].between(low_stack_size_var, high_stack_size_var)]
                     # if player_parse_var == 'Specific' and player_names:
                     #     working_df = working_df[working_df['BaseName'].isin(player_names)]