variable_get('...', array()) leads to "dynamic value..." in config, .install

A `variable_get()` that is initialized from an empty array, e.g., `variable_get('mymodule_setting', array())`, leads to a `"dynamic value in file..."` value in the config file and install function. Since empty arrays are relatively common initialization values, it would be nice to catch this situation and properly set the config and install values.