Add Checker Framework stub file for Guava (fixes #920).

This commit adds a stub file as described in
https://checkerframework.org/manual/#stub.  The initial file contains
annotations for com.google.common.base.Strings.isNullOrEmpty.
diff --git a/build.gradle b/build.gradle
index 3d8e1c3..af4ff26 100644
--- a/build.gradle
+++ b/build.gradle
@@ -96,7 +96,9 @@
         }
         if (useCheckerFramework) {
             it.options.compilerArgs += [
-                '-processor', 'com.google.auto.value.processor.AutoValueProcessor,org.checkerframework.checker.nullness.NullnessChecker'
+                '-processor',
+		'com.google.auto.value.processor.AutoValueProcessor,org.checkerframework.checker.nullness.NullnessChecker',
+		"-Astubs=$rootDir/checker-framework/stubs"
             ]
         }
         it.options.encoding = "UTF-8"