Fix NPE in ExtractorMediaPeriod

Also turn on nullity checks for ExtractorMediaPeriod.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=208467644

library/core/src/main/java/com/google/android/exoplayer2/util/MimeTypes.java

@@ -123,7 +123,7 @@ public final class MimeTypes {
    * @param mimeType The mimeType to test.
    * @return Whether the top level type is audio.
    */
-  public static boolean isAudio(String mimeType) {
+  public static boolean isAudio(@Nullable String mimeType) {
     return BASE_TYPE_AUDIO.equals(getTopLevelType(mimeType));
   }
 
@@ -133,7 +133,7 @@ public final class MimeTypes {
    * @param mimeType The mimeType to test.
    * @return Whether the top level type is video.
    */
-  public static boolean isVideo(String mimeType) {
+  public static boolean isVideo(@Nullable String mimeType) {
     return BASE_TYPE_VIDEO.equals(getTopLevelType(mimeType));
   }
 
@@ -143,7 +143,7 @@ public final class MimeTypes {
    * @param mimeType The mimeType to test.
    * @return Whether the top level type is text.
    */
-  public static boolean isText(String mimeType) {
+  public static boolean isText(@Nullable String mimeType) {
     return BASE_TYPE_TEXT.equals(getTopLevelType(mimeType));
   }
 
@@ -153,7 +153,7 @@ public final class MimeTypes {
    * @param mimeType The mimeType to test.
    * @return Whether the top level type is application.
    */
-  public static boolean isApplication(String mimeType) {
+  public static boolean isApplication(@Nullable String mimeType) {
     return BASE_TYPE_APPLICATION.equals(getTopLevelType(mimeType));
   }
 

library/core/src/main/java/com/google/android/exoplayer2/util/Util.java

@@ -73,6 +73,7 @@ import java.util.regex.Pattern;
 import java.util.zip.DataFormatException;
 import java.util.zip.Inflater;
 import org.checkerframework.checker.initialization.qual.UnknownInitialization;
+import org.checkerframework.checker.nullness.compatqual.NullableType;
 import org.checkerframework.checker.nullness.qual.EnsuresNonNull;
 import org.checkerframework.checker.nullness.qual.PolyNull;
 
@@ -271,6 +272,13 @@ public final class Util {
     return value;
   }
 
+  /** Casts a nullable type array to a non-null type array without runtime null check. */
+  @SuppressWarnings({"contracts.postcondition.not.satisfied", "return.type.incompatible"})
+  @EnsuresNonNull("#1")
+  public static <T> T[] castNonNullTypeArray(@NullableType T[] value) {
+    return value;
+  }
+
   /**
    * Copies and optionally truncates an array. Prevents null array elements created by {@link
    * Arrays#copyOf(Object[], int)} by ensuring the new length does not exceed the current length.