MODE-2178 Added test that verifies that the existing behavior works as expected

Using a criteria "`... WHERE [car:engine] == '' ...`" will only work when the property value
is non-null and is an empty string. This works as expected. 

Note that using 
```
  ... WHERE LENGTH([car:engine]) == 0
```
or
```
  ... WHERE [car:engine] IS NULL OR [car:engine] = ''
```
both work fine even before this commit.