Treat null values and empty strings the same way.

[joohokim1]

Is your feature request related to a problem? Please describe.
Recently, I've received a question that why the empty strings are shown as mismatched not as missing.
My answer was that they are diffent as a perspective of a programmer, but she said it's very confusing.
Besides, I found out a similar product also treats the two values as the same.
So, I realized that normal users do not want to distinguish those two.

Describe the solution you'd like
Show empty strings as missing in Histogram (above the grid of data preparation edit page).
The grid itself has no change.

Additional context
I was going to solve this issue as below:
1. Show null values as empty string in UI. (null) will not be shown on grid any more.
2. Treat empty string as missing in histogram bar.
3. Internal corresponding code change.

NULL vs Empty String is not finished yet.
I want to talk about this matter with other contributors.
If needed, let's open an issue about this tiresome but important thing.

This issue must be done before #2241.

[joohokim1]

Integrated test #1 passed.