Use @nullable on public *methods* consistently for Guava classes of common.base

(plus StringUtil, given how erratic its null-friendliness is). Of course,
nothing is *proving* that these are correct or complete (and probably nothing
short of moving to full JSR308ness can?).
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=70026461

Author: cgruber

guava/src/com/google/common/base/Defaults.java

@@ -22,6 +22,8 @@
 import java.util.HashMap;
 import java.util.Map;
 
+import javax.annotation.Nullable;
+
 /**
  * This class provides default values for all Java types, as defined by the JLS.
  *
@@ -56,6 +58,7 @@ private static <T> void put(Map<Class<?>, Object> map, Class<T> type, T value) {
    * false} for {@code boolean} and {@code '\0'} for {@code char}. For non-primitive types and
    * {@code void}, null is returned.
    */
+  @Nullable
   public static <T> T defaultValue(Class<T> type) {
     // Primitives.wrap(type).cast(...) would avoid the warning, but we can't use that from here
     @SuppressWarnings("unchecked") // the put method enforces this key-value relationship

guava/src/com/google/common/base/FinalizableReferenceQueue.java

@@ -30,6 +30,8 @@
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
+import javax.annotation.Nullable;
+
 /**
  * A reference queue with an associated background thread that dequeues references and invokes
  * {@link FinalizableReference#finalizeReferent()} on them.
@@ -228,6 +230,7 @@ interface FinalizerLoader {
      *
      * @throws SecurityException if we don't have the appropriate privileges
      */
+    @Nullable
     Class<?> loadFinalizer();
   }
 

guava/src/com/google/common/base/StandardSystemProperty.java

@@ -19,6 +19,8 @@
 import com.google.common.annotations.Beta;
 import com.google.common.annotations.GwtIncompatible;
 
+import javax.annotation.Nullable;
+
 /**
  * Represents a {@linkplain System#getProperties() standard system property}.
  *
@@ -130,6 +132,7 @@ public String key() {
    * Returns the current value for this system property by delegating to
    * {@link System#getProperty(String)}.
    */
+  @Nullable
   public String value() {
     return System.getProperty(key);
   }

guava/src/com/google/common/base/Strings.java

@@ -54,7 +54,8 @@ public static String nullToEmpty(@Nullable String string) {
    * @return {@code string} itself if it is nonempty; {@code null} if it is
    *     empty or null
    */
-  public static @Nullable String emptyToNull(@Nullable String string) {
+  @Nullable
+  public static String emptyToNull(@Nullable String string) {
     return isNullOrEmpty(string) ? null : string;
   }