Prior to this commit, if MockServletContext was configured with a
FileSystemResourceLoader, invocations of the following methods on a
Microsoft Windows operating system resulted in an InvalidPathException
if the supplied path contained a colon (such as "C:\\temp"). This is
inconsistent with the behavior on non-Windows operating systems. In
addition, for comparable errors resulting in an IOException, those
methods (except getRealPath()) return null instead of throwing the
exception.

- getResourcePaths()
- getResource()
- getResourceAsStream()
- getRealPath()

This commit makes handling of InvalidPathException and IOException
consistent for these methods: both exceptions now result in null be
returned by these methods.

Closes gh-23717