Add @Nullable annotations

[m15o]

• Added @Nullable annotations to methods and fields in micrometer-tracing-bridges directories.
• Added @Nullable annotations to TraceContext.sampled() and Tracer.nextSpan(Span), since the documents say the functions might return `null.

Fixes #732

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FinishedSpan.getKind() is not annotated with @Nullable, but its implementetion might return null. I am no sure whether the implementation or the interface was wrong, so I didn't fix it.

tracing/micrometer-tracing/src/main/java/io/micrometer/tracing/exporter/FinishedSpan.java

Lines 181 to 184 in 85a614a

	/**
	* @return span's kind
	*/
	Span.Kind getKind();

tracing/micrometer-tracing-bridges/micrometer-tracing-bridge-otel/src/main/java/io/micrometer/tracing/otel/bridge/OtelFinishedSpan.java

Lines 259 to 261 in bc290c9

	public Span.Kind getKind() {
	if (this.spanData.getKind() == SpanKind.INTERNAL) {
	return null;

tracing/micrometer-tracing-bridges/micrometer-tracing-bridge-brave/src/main/java/io/micrometer/tracing/brave/bridge/BraveFinishedSpan.java

Lines 168 to 170 in d1383ae

	public Span.Kind getKind() {
	if (this.mutableSpan.kind() == null) {
	return null;