[docs] Fix warnings about undocumented default constructors (#6151)

wpilibj/src/main/java/edu/wpi/first/wpilibj/DMASample.java

@@ -41,6 +41,9 @@ public class DMASample {
 
   private final DMAJNISample m_dmaSample = new DMAJNISample();
 
+  /** Default constructor. */
+  public DMASample() {}
+
   public DMAReadStatus update(DMA dma, double timeoutSeconds) {
     return DMAReadStatus.getValue(m_dmaSample.update(dma.m_dmaHandle, timeoutSeconds));
   }

wpilibj/src/main/java/edu/wpi/first/wpilibj/DigitalSource.java

@@ -11,6 +11,9 @@ package edu.wpi.first.wpilibj;
  * source. The source can either be a digital input or analog trigger but not both.
  */
 public abstract class DigitalSource implements AutoCloseable {
+  /** Default constructor. */
+  public DigitalSource() {}
+
   /**
    * Returns true if this DigitalSource is an AnalogTrigger.
    *

wpilibj/src/main/java/edu/wpi/first/wpilibj/event/EventLoop.java

@@ -15,6 +15,9 @@ public final class EventLoop {
   private final Collection<Runnable> m_bindings = new LinkedHashSet<>();
   private boolean m_running;
 
+  /** Default constructor. */
+  public EventLoop() {}
+
   /**
    * Bind a new action to run when the loop is polled.
    *

wpilibj/src/main/java/edu/wpi/first/wpilibj/smartdashboard/SendableBuilderImpl.java

@@ -109,6 +109,9 @@ public class SendableBuilderImpl implements NTSendableBuilder {
 
   private final List<AutoCloseable> m_closeables = new ArrayList<>();
 
+  /** Default constructor. */
+  public SendableBuilderImpl() {}
+
   @Override
   @SuppressWarnings("PMD.AvoidCatchingGenericException")
   public void close() {