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Blank data can be passed to both, Static as well as Data sheet parameters.

Static

Use open and close double quotes with space inside it (" ") to pass a blank value as static data. 

Data Sheet Parameter

Enter $NULL$ in the data sheet for the required parameter to pass a data sheet parameter as blank value. By default, the first time a user opens the data sheet, $NULL$ will be displayed for all parameters.

• You must not execute test cases with the default data sheet values. The data sheet is required to be opened and attached at least once prior to execution.
• An empty cell in the data sheet acts as a break point for tasks and should be used for passing blank values to action parameters.   

See also,

• Creating a Test Case
• Updating Test Data
• Editing a Test Case
• Searching a Test Case
• Removing a Test Case
• Sync Test Case
• Critical Data Protection