Add test for missing values in diagnostics

* Henchman version: 0.0.3

### Description

The following lines loops are not run during tests

+ `if duplicates.shape[0] > 0:` (line 59)
+ `if datacol.isnull().sum() > 0`
   * Line 136 for the object column summary
   * Line 161 for boolean 
   * Line 177 for numeric

