Make unset=null part of the interface of lastYieldedValue

src/main/java/com/team766/framework/LaunchedContextWithValue.java

@@ -7,6 +7,8 @@
 public interface LaunchedContextWithValue<T> extends LaunchedContext {
     /**
      * Return the most recent value that the Procedure passed to Context.yield(T).
+     * Return null if a value has never been yielded, or a value has not been yielded since the last
+     * call to getAndClearLastYieldedValue.
      */
     T lastYieldedValue();
 

src/test/java/com/team766/framework/YieldWithValueTest.java

@@ -12,6 +12,11 @@ private static class ValueConsumer extends Procedure {
         @Override
         public void run(Context context) {
             var generator = context.startAsync(new ValueGenerator());
+
+            assertNull(
+                    "lastYieldedValue should be null before the procedure yields a value",
+                    generator.lastYieldedValue());
+
             while (generator.lastYieldedValue() == null || generator.lastYieldedValue() < 10) {
                 var value = generator.lastYieldedValue();
                 if (value != null) {