Prior to this commit, calls to setLocale() MockHttpServletResponse
would result in a NullPointerException if the supplied Locale was null.
Although the Javadoc for setLocale(Locale) and addHeader(String, String)
in javax.servlet.ServletResponse does not specify how a null
Locale should be handled, both Tomcat and Jetty simply ignore a null
value.
This commit therefore updates MockHttpServletResponse to silently
ignore a null Locale passed to setLocale().
Closes gh-26493
c19b2851f1