!empty() check before a foreach statement
The loop won't be executed if $terms is false/null/an empty array.

As such: you don't need the empty check. It doesn't hurt, but it's simply not required.

If $terms is sometimes not an array

You can just cast it to an array before use:

$terms = get_the_terms( $postid, 'fecha' );
foreach((array) $terms as $term) {
    $myterm = $term-&gt;slug;
    if(in_array($myterm, $queried_terms)) {
        continue;
    }
    $queried_terms[] = $myterm;
}


The code would work with or without casting to an array - the only difference is that if $terms is not an array, a notice would be thrown without it. 

if this is part of another function

you can simply return-early to keep your code concise:

function foo() {
    $terms = get_the_terms( $postid, 'fecha' );
    if (!$terms) {
        return array();
    }

    $queried_terms = array();
    foreach($terms as $term) {
        $myterm = $term-&gt;slug;
        if(in_array($myterm, $queried_terms)) {
            continue;
        }
        $queried_terms[] = $myterm;
    }
    return $queried_terms;
}


But, any way you slice it - the empty check isn't strictly required, and deeper code nesting is something to be aware of and avoid.