Create a base class for DASH downloading related tests

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=158129802

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java

@@ -110,7 +110,8 @@ public final class SimpleCache implements Cache {
   @Override
   public synchronized NavigableSet<CacheSpan> getCachedSpans(String key) {
     CachedContent cachedContent = index.get(key);
-    return cachedContent == null ? null : new TreeSet<CacheSpan>(cachedContent.getSpans());
+    return cachedContent == null || cachedContent.isEmpty() ? null
+        : new TreeSet<CacheSpan>(cachedContent.getSpans());
   }
 
   @Override