Move handling of trivial cases from internal methods with span
arguments to to the very beginning of public operators. This avoids
creating unneeded spans, checking them to empty state and some other
unneeded operations.

Does not affect time of processing non-trivial arguments.

Discussion: dotnet#84721

Address PR feedback.

… why. To avoid that we can make an error message out of the comment. So when it fails, it's printed.