Improper varargs check in KeyValues and Tags

[jonatan-ivanov]

If I do this:

KeyValue kv = null;
KeyValues.of("a", "1").and(kv);

I get an NPE since in case of varargs:

KeyValues and(@Nullable KeyValue... keyValues) {

this is not a proper check:

if (keyValues == null || keyValues.length == 0)

KeyValues.of(null) will result in an empty keyValues array that has one null element in the and method which passes the check.
Maybe this can be just a documentation issue and we should state that null elements are not allowed without checking.

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.lang.Comparable.compareTo(Object)" because "a[runHi]" is null
	at java.base/java.util.ComparableTimSort.countRunAndMakeAscending(ComparableTimSort.java:320)
	at java.base/java.util.ComparableTimSort.sort(ComparableTimSort.java:188)
	at java.base/java.util.Arrays.sort(Arrays.java:1041)
	at io.micrometer.common.KeyValues.<init>(KeyValues.java:47)
	at io.micrometer.common.KeyValues.and(KeyValues.java:107)
	at io.micrometer.core.samples.Sample.main(Sample.java:24)

Since KeyValues is basically a copy of Tags, Tags has the same issue.

Discovered based on #3849 by @dietzsch