make SafetySettings#method nullable

Author: David Motsonashvili

firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/internal/util/conversions.kt

@@ -100,7 +100,7 @@ internal fun SafetySetting.toInternal() =
   com.google.firebase.vertexai.common.shared.SafetySetting(
     harmCategory.toInternal(),
     threshold.toInternal(),
-    method.toInternal()
+    method?.toInternal()
   )
 
 internal fun makeMissingCaseException(source: String, ordinal: Int): SerializationException {

firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/type/SafetySetting.kt

@@ -22,10 +22,11 @@ package com.google.firebase.vertexai.type
  *
  * @param harmCategory The relevant [HarmCategory].
  * @param threshold The threshold form harm allowable.
- * @param method Specify if the threshold is used for probability or severity score.
+ * @param method Specify if the threshold is used for probability or severity score, if not
+ * specified it will default to [HarmBlockMethod.PROBABILITY].
  */
 public class SafetySetting(
   internal val harmCategory: HarmCategory,
   internal val threshold: HarmBlockThreshold,
-  internal val method: HarmBlockMethod = HarmBlockMethod.PROBABILITY
+  internal val method: HarmBlockMethod? = null,
 )