Fixup spotbugs warnings.

Author: nedtwigg

lib/src/main/java/com/diffplug/spotless/FileSignature.java

@@ -103,7 +103,7 @@ public static class Promised implements Serializable {
 		@SuppressFBWarnings("SE_TRANSIENT_FIELD_NOT_RESTORED")
 		private transient @Nullable FileSignature cached;
 
-		private Promised(List<File> files, FileSignature cached) {
+		private Promised(List<File> files, @Nullable FileSignature cached) {
 			this.files = files;
 			this.cached = cached;
 		}

lib/src/main/java/com/diffplug/spotless/FormatterStepSerializationRoundtrip.java

@@ -22,6 +22,7 @@
 import edu.umd.cs.findbugs.annotations.Nullable;
 
 class FormatterStepSerializationRoundtrip<RoundtripState extends Serializable, EqualityState extends Serializable> extends FormatterStepEqualityOnStateSerialization<EqualityState> {
+	private static final long serialVersionUID = 1L;
 	private final String name;
 	private final transient ThrowingEx.Supplier<RoundtripState> initializer;
 	private @Nullable RoundtripState roundtripStateInternal;