Set a default empty array if value is missing

glean_parser/templates/rust.jinja2

@@ -24,7 +24,7 @@ Jinja2 template is not. Please file bugs! #}
           #[serde(skip_serializing_if = "Option::is_none")]
           pub {{itemname|snake_case}}: Option<{{ name ~ "Item" ~ itemname|Camelize ~ "Object" }}>,
           {% elif val.type == "array" %}
-          #[serde(skip_serializing_if = "Vec::is_empty")]
+          #[serde(skip_serializing_if = "Vec::is_empty", default = "Vec::new")]
           pub {{itemname|snake_case}}: {{ name ~ "Item" ~ itemname|Camelize }},
           {% else %}
           #[serde(skip_serializing_if = "Option::is_none")]