How to test for blank text field when using robotframework-selenium?
You can use either a single backslash \ or special variable ${EMPTY} to create an empty string in the test data. User guide has the details: Robot Framework User Guide.
${EMPTY} is good for a blank value but, surprisingly, it didn't work for an empty value.
I found what I was looking for. The field I was verifying had no value in its value attribute and I wanted to verify it. It was returning '' as the value and when using ${EMPTY} it couldn't find '''' instead. Such a minor thing but ended up solving what I needed, so it depends what you're seeking to verify.