file(GENERATE): Restore INPUT|CONTENT parse checking

Refactoring in commit bff468c9 (cmFileCommand: Use cm::optional for
keyword argument presence, 2022-06-30, v3.25.0-rc1~512^2) accidentally
broke the check that the input argument is either `INPUT` or `CONTENT`.

The check is supposed to fail when arguments are passed in the wrong
order.  For example:

    file(GENERATE OUTPUT ...
         TARGET <target>
         CONTENT <content>)

Prior to this fix, the input method would be CONTENT, but because the
first parsed keyword is not `CONTENT`, `inputIsContent` would be false.
The first parsed keyword isn't INPUT either, so we would not continue
into the error condition. CMake would then try to handle this as an
input file, when there isn't one, resulting in uninitialized memory
usage and segfaults or corruption later on.

Fixes: #25169