refactor: remove AnyDuringMigration from input (#6310)

* fix: AnyDuringMigration for input

* refactor: fix type of getSourceBlock()

core/input.ts

@@ -63,7 +63,7 @@ export class Input {
    * Get the source block for this input.
    * @return The source block, or null if there is none.
    */
-  getSourceBlock(): Block|null {
+  getSourceBlock(): Block {
     return this.sourceBlock_;
   }
 
@@ -290,9 +290,6 @@ export class Input {
     if (this.connection) {
       this.connection.dispose();
     }
-    // AnyDuringMigration because:  Type 'null' is not assignable to type
-    // 'Block'.
-    this.sourceBlock_ = null as AnyDuringMigration;
   }
 }