I may be missing something here but the Knime 2.5.2 Missing Value node lets you convert a string column with the missing value ? to a null string by setting the default column for String Values to Fixed Value with nothing in the value box to the right.
It is the issue of dealing with empty strings which is the issue, not the issue of dealing with missing values which is well supported.
so having a cell with an empty string is very hard to do anything with. I.e. filtering out the row is difficult, and replacing the empty string for another value is tricky too.
I am using KNIME 2.7.1 and I am still experiencing issues with empty strings. I am currently using Java Snippets to replace the values, yet this feels more like a workaround than a solution. I am collaborating with non-developers on the workflows, so using Java Snippets is not an option.
I was expecting the Missing Values node to replace empty strings, instead I found that this works only with special '?' values as mentioned above, which I'm not sure how I am meant to create. My data is coming in from a Database Reader node, so if there is any way to make this produce a '?' instead of an empty string, this should work.
I found "Missing value" node very useful. This will help set value of any String, Integer or Double cells with a default value. Not only that it will help map mean, minimum, etc... Wonderful tool.....:)