hive_generator: basic nullability awareness

* Only for built_value classes for reducing the amount of generated code
to a reasonable level

Author: KalilDev

hive_generator/lib/src/class_builder.dart

@@ -130,6 +130,7 @@ class ClassBuilder extends _ClassBuilderBase {
         field.type,
         'fields[${field.index}]',
         nestedBuilders: field.nestedBuilders,
+        nullable: field.isNullable,
       )}');
     }
 
@@ -141,6 +142,7 @@ class ClassBuilder extends _ClassBuilderBase {
     DartType type,
     String variable, {
     bool nestedBuilders,
+    bool nullable = true,
   }) {
     String builderConstructor;
     String typeToBeCasted;
@@ -182,29 +184,40 @@ class ClassBuilder extends _ClassBuilderBase {
         '($castedVariable)'
         '${shouldBeBuilt ? '.build()' : ''}';
 
+    if (!nullable) {
+      return buildExpression;
+    }
+
     return '$variable == null ? null : $buildExpression';
   }
 
   @override
   String cast(
     DartType type,
-    String variable, [
-    bool nestedBuilders = false,
-  ]) {
+    String variable, {
+    bool nestedBuilders,
+    bool nullable = true,
+  }) {
     if (!isBuiltOrBuiltCollection(type) &&
         !isBuilderOrCollectionBuilder(type)) {
       // This value needs no special treatment.
       return super.cast(type, variable);
     }
 
-    if ((isBuilt(type) && nestedBuilders) || isBuilder(type)) {
+    if ((isBuilt(type) && nestedBuilders == true) || isBuilder(type)) {
       // We need to call .toBuilder(), because variable is always an Built
       // value, but we need an Builder value.
-      return '($variable as ${_displayString(type)})?.toBuilder()';
+      final toBuilder = '${nullable ? '?' : ''}.toBuilder()';
+      return '($variable as ${_displayString(type)})$toBuilder';
     }
 
     if (isBuiltCollection(type) || isCollectionBuilder(type)) {
-      return _castBuiltCollection(type, variable, nestedBuilders ?? false);
+      return _castBuiltCollection(
+        type,
+        variable,
+        nestedBuilders: nestedBuilders,
+        nullable: nullable ?? true,
+      );
     }
 
     // We just need to cast the value. This happens when the type is of a Built