Data export: Change firstOpened and committed default value to missing value

As mentioned previously, the default value for the timestamp variables firstOpened and committed columns in the data export would be easier to parse if it was the default missing value (empty string~~, I assume~~) rather than FALSE to avoid mixing of types (timestamp and boolean).

Researchers would still be able to infer, that rows with missing values indicate that a survey has not yet been opened / committed, since this is a value that is set by the system and thus would not be confused for genuinely missing data.

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Replace inital FALSE values for firstOpened and committed columns, with an empty string ''