Fix null check in withStringMatcher

[kdomo]

TypedExampleMatcher.withStringMatcher(StringMatcher) asserted the ignoredPaths field instead of its defaultStringMatcher parameter:

public ExampleMatcher withStringMatcher(StringMatcher defaultStringMatcher) {
    Assert.notNull(ignoredPaths, "DefaultStringMatcher must not be empty");
    ...
}

ignoredPaths is always set, so the assertion always passed and a null defaultStringMatcher was accepted and stored, only to fail later. This asserts the parameter instead, matching the message and the sibling withIgnorePaths(String...) method, and adds a test covering the null argument.

Closes #3539