Support additional URI Template Levels

In versions of Feign before 10.x, if the URI template looked like http://base/action/{arg1}/{arg2}, and null was passed as the value to arg1, the URI

http://base/action/{arg1}/somevalue

would be tried, and rejected with IllegalArgumentException. That probably wasn't the right thing to do, but NOW, the null is actually resolved, and the situation is actually worse, resulting in the URI

http://base/action//someValue

resulting in, if you are using Spring Boot 2, a "not normalized URI exception" because they are doing security scans on URIs and disallowing consecutive slashes.

Even worse, if a value is passed to arg1 and null is passed to arg 2 and it is resolved, it can map to the entirely wrong controller method as:

http://base/action/someValue/

if these are GET operations, that might get the entire list associated with someValue, instead of the intended one value "someValue2".

Shouldn't feign refuse to resolve PATH variables if the supplied argument is null, and just throw some kind of NotNullable exception?

No source file or test is named. Start by reproducing URI-template expansion with null path arguments, then locate the Java path-variable resolution entry point and related tests. Done means null path variables are rejected consistently rather than producing consecutive or trailing slashes, with the behavior covered by a regression test.