Use Float.NaN for Cue#DIMEN_UNSET

Float.MIN_VALUE is very close to zero:
"A constant holding the smallest positive nonzero value of type float"
https://developer.android.com/reference/java/lang/Float.html#MIN_VALUE

Float.NaN more clearly conveys this is an invalid number,
and will also more clearly carry over if we do maths on it accidentally.

PiperOrigin-RevId: 276277784

library/core/src/main/java/com/google/android/exoplayer2/text/Cue.java

@@ -33,7 +33,8 @@ public class Cue {
   public static final Cue EMPTY = new Cue("");
 
   /** An unset position or width. */
-  public static final float DIMEN_UNSET = Float.MIN_VALUE;
+  // Note: We deliberately don't use Float.MIN_VALUE because it's positive & very close to zero.
+  public static final float DIMEN_UNSET = Float.NaN;
 
   /**
    * The type of anchor, which may be unset. One of {@link #TYPE_UNSET}, {@link #ANCHOR_TYPE_START},