fix: strip non-nullable optional params in Dart/Flutter SDKs

This change updates the Dart and Flutter SDK templates to conditionally
include non-nullable optional parameters in the request params map only
when they are not null.

Previously, all parameters were always included in the params map,
which could lead to unnecessary null values being sent in API requests.

This implementation matches the Python SDK behavior (templates/python/base/params.twig)
where optional non-nullable parameters are checked with "if not None" before
being added to api_params.

Changes:
- Modified templates/dart/base/utils.twig map_parameter macro
- Modified templates/flutter/base/utils.twig map_parameter macro
- Uses Dart's collection-if syntax for conditional parameter inclusion
- Adds null assertion operator (!) for enum values when inside null check

Example generated code:
```dart
final Map<String, dynamic> apiParams = {
  'requiredParam': requiredParam,
  if (optionalParam != null) 'optionalParam': optionalParam,
};
```

Author: ChiragAgg5k

templates/dart/base/utils.twig

@@ -1,6 +1,10 @@
 {% macro map_parameter(parameters) %}
 {% for parameter in parameters %}
+{% if not parameter.nullable and not parameter.required %}
+if ({{ parameter.name | caseCamel | overrideIdentifier }} != null) '{{ parameter.name }}': {{ parameter.name | caseCamel | overrideIdentifier }}{% if parameter.enumValues | length > 0  %}!{% endif %}{% if parameter.enumValues | length > 0  %}.value{% endif %},
+{% else %}
 '{{ parameter.name }}': {{ parameter.name | caseCamel | overrideIdentifier }}{% if parameter.enumValues | length > 0  %}{% if not parameter.required %}?{% endif %}.value{% endif %},
+{% endif %}
 {% endfor %}
 {% endmacro %}
 

templates/flutter/base/utils.twig

@@ -1,6 +1,10 @@
 {%- macro map_parameter(parameters) -%}
 {%- for parameter in parameters ~%}
+{% if not parameter.nullable and not parameter.required %}
+            if ({{ parameter.name | caseCamel | overrideIdentifier }} != null) '{{ parameter.name }}': {{ parameter.name | caseCamel | overrideIdentifier }}{% if parameter.enumValues | length > 0  %}!{% endif %}{% if parameter.enumValues | length > 0  %}.value{% endif %},
+{% else %}
             '{{ parameter.name }}': {{ parameter.name | caseCamel | overrideIdentifier }}{% if parameter.enumValues | length > 0  %}{% if not parameter.required %}?{% endif %}.value{% endif %},
+{% endif %}
 {%- endfor ~%}
 {%- endmacro ~%}