Use !empty() instead of isset() to check for entity language

In entity_metadata_field_get_language() the default language is determined using the following:
$default_langcode = isset($entity->language) ? $entity->language : LANGUAGE_NONE;

However, if no language is set for the entity, $entity->language may be set to an empty string (''). This causes isset() to return TRUE and ends up returning an empty string if $langcode == LANGUAGE_NONE due to the following a few lines lower:
$langcode = ($langcode != LANGUAGE_NONE) ? field_valid_language($langcode, $default_langcode) : $default_langcode;

The line to determine the the default language should use !empty() instead, this will cover situations where it is not set as well as situations where it is set to NULL or an empty string.