Avoid allocation of iterator with Tags#empty instance

[shakuzen]

Special case calling Tags.and and Tags.of on a Tags.empty() instance to avoid instantiating an iterator for the EMPTY constant static instance.
I've checked this change locally with a JMH test and profiling. This change avoids unnecessary allocation in e.g. the following code:

Tags extraTags = Tags.empty();
Tags anotherTag = Tags.of("a", "b");
Tags combined = anotherTag.and(extraTags);
combined = Tags.concat(extraTags, "a", "b");

This shows up in several places in our instrumentation where we allow passing extraTags to add to meters created by the instrumentation, and the default is no extra tags (Tags.empty()).

[shakuzen]

I've added tests verifying there are now no allocations when using and or of with the empty Tags instance. These tests may be re-worked in the future with #3209.