If you are considering notes that start with whitespace, i.e., not trimming the spaces in between characters. Then you can use this starts-with function to query for those that does not start with a whitespace.
Please see documentation for this function
In Xpath, only available option relevant to this scenario are these functions. Please look through and see if you are able to figure out a solution.
All functions list
If you can somehow manipulate your data at the time it is stored in database, like I described in above comment that leading and trailing whitespaces are trimmed using trim function in a microflow when data is committed. In that case, the not(starts-with) function will work fine