Add method in KiwiPreconditions to check collection contains only non-null elements

[sleberknight]

Add a new validation method in KiwiPreconditions that checks a Collection argument is not empty, and that none of its elements are null.

Some possible names:

• checkAllElementsNotNull
• checkArgumentAllElementsNotNull
• checkArgumentContainsOnlyNotNull
• checkArgumentContainsOnlyNotNullElements

To be consistent, it should probably start with checkArgument since that is our general convention in KiwiPreconditions.

As I write this, I like checkArgumentAllElementsNotNull and checkArgumentContainsOnlyNotNull best, since they are not overly long and (seem to) convey the intent.

[sleberknight]

I came across this requirement recently in some code that was validating a Set<String> named typesToMatch and did it in two steps:

checkArgumentNotEmpty(collection, "typesToMatch must not be empty");
checkArgument(typesToMatch.stream().noneMatch(StringUtils::isBlank),
        "typesToMatch must not contain blank elements");

This is probably not the most efficient since noneMatch must traverse the entire collection instead of short-circuiting at the first null element. After writing this, I took a few moments and went back to that code and changed it to:

checkArgumentNotEmpty(collection, "typesToMatch must not be empty");
var anyBlank = typesToMatch.stream().anyMatch(StringUtils::isBlank);
checkArgument(!anyBlank, "typesToMatch must not contain blank elements");

This new implementation using anyMatch stops once it finds the first blank String.

Since this was validating a collection of String it uses a check for a blank string as opposed to null. Should there be a dedicated method to validate a collection of String contains all non-blank elements? e.g., checkArgumentContainsOnlyNotBlank which only accepts Collection<String>.