You don’t need to care.
The warning enacts a standard of cleanliness of text files in regard to whitespace, the kind of thing that many programmers tend to care about. As the manual explains:
What are considered whitespace errors is controlled
by core.whitespace configuration. By default, trailing whitespaces
(including lines that solely consist of whitespaces) and a space
character that is immediately followed by a tab character inside the
initial indent of the line are considered whitespace errors.By default, the command outputs warning messages but applies the patch.
So, the “error” means that the change introduces a trailing whitespace, a whitespace-only line, or a space that precedes a tab. Other than that fact, there is nothing erroneous about the change, and it will apply cleanly and correctly. In other words, if you don’t care about the “incorrect” whitespace, feel free to ignore the warning or turn it off with git config apply.whitespace nowarn.