fix(crystal): make optional parameters truly optional

Author: n-rodriguez

modules/openapi-generator/src/main/resources/crystal/partial_model_generic.mustache

@@ -89,7 +89,7 @@
     {{/discriminator}}
     # Initializes the object
     # @param [Hash] attributes Model attributes in the form of hash
-    def initialize({{#requiredVars}}@{{{name}}} : {{{dataType}}}{{^-last}}, {{/-last}}{{/requiredVars}}{{#hasRequired}}{{#hasOptional}}, {{/hasOptional}}{{/hasRequired}}{{#optionalVars}}@{{{name}}} : {{{dataType}}}?{{^-last}}, {{/-last}}{{/optionalVars}})
+    def initialize({{#requiredVars}}@{{{name}}} : {{{dataType}}}{{^-last}}, {{/-last}}{{/requiredVars}}{{#hasRequired}}{{#hasOptional}}, {{/hasOptional}}{{/hasRequired}}{{#optionalVars}}@{{{name}}} : {{{dataType}}}? = nil{{^-last}}, {{/-last}}{{/optionalVars}})
     end
 
     # Show invalid properties with the reasons. Usually used together with valid?