DT[f.A == "", :] is bugged for columns with all empty strings

```Python
from datatable import dt, f

DT = dt.Frame({"A": ["", ""]})
DT[f.A == "", dt.count()][0, 0]
# 0
```

If any value in the column is not an empty string, it works as expected.

Workaround:

```Python
DT[dt.str.len(f.A) == 0, dt.count()][0, 0]
# 2
```