Correctly handle reading of an empty string without a terminating NUL byte

Issue: #2152

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=141159330

library/src/main/java/com/google/android/exoplayer2/util/ParsableByteArray.java

@@ -431,6 +431,9 @@ public final class ParsableByteArray {
    * @return The string encoded by the bytes.
    */
   public String readNullTerminatedString(int length) {
+    if (length == 0) {
+      return "";
+    }
     int stringLength = length;
     int lastIndex = position + length - 1;
     if (lastIndex < limit && data[lastIndex] == 0) {