Made maxLines property nullable and added assertions (#810)

* Made maxLines property nullable and added assertions

maxLines must be nullable to be able to expand vertically on user input

it also needs to be nullable to use the expands property without errors

* Fixed error cause by null operator

Author: S-ecki

lib/src/fields/form_builder_text_field.dart

@@ -68,7 +68,7 @@ class FormBuilderTextField extends FormBuilderField<String> {
   final bool enableSuggestions;
 
   /// {@macro flutter.widgets.editableText.maxLines}
-  final int maxLines;
+  final int? maxLines;
 
   /// {@macro flutter.widgets.editableText.minLines}
   final int? minLines;
@@ -343,12 +343,13 @@ class FormBuilderTextField extends FormBuilderField<String> {
     this.mouseCursor,
   })  : assert(initialValue == null || controller == null),
         assert(minLines == null || minLines > 0),
+        assert(maxLines == null || maxLines > 0),
         assert(
-          (minLines == null) || (maxLines >= minLines),
+          (minLines == null) || (maxLines == null) || (maxLines >= minLines),
           'minLines can\'t be greater than maxLines',
         ),
         assert(
-          !expands || (minLines == null),
+          !expands || (minLines == null && maxLines == null),
           'minLines and maxLines must be null when expands is true.',
         ),
         assert(!obscureText || maxLines == 1,