public FormStateInterface::isValueEmpty($key)
Determines if a specific key has a value in the submitted form values.
string|array $key: Values are stored as a multi-dimensional associative array. If $key is a string, it will return empty($values[$key]). If $key is an array, each element of the array will be used as a nested key. If $key = array('foo', 'bar') it will return empty($values['foo']['bar']).
bool TRUE if the $key has no value, FALSE otherwise.
public function isValueEmpty($key);
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