BUG: Check for null values when infering excel in read_clipboard
GH41108
Stripping whitespace on read_clipboard causes data copied from
excel to lose the tab separator between the first column and
the nexts, shifting the data into the wrong column

This just removes the .lstrip() except for the first line
containing headers, to avoid breaking header-less index in csv
files

- [x] closes #41108
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry
