Perhaps something like so:
<?php if( !empty( get_field('birthday') ) ): ?>
<div class="form-question">
<?php echo 'Birthday:'; ?>
</div>
<div class="form-answer">
<?php the_field('birthday'); ?>
</div>
<?php endif; ?>
Or maybe you need to display the .form-question divs:
<div class="form-question">
<?php echo get_field('birthday') ? 'Birthday:' : ""; ?>
</div>
<div class="form-answer">
<?php the_field('birthday'); ?>
</div>
Just in case this looks a bit strange to you, there’s a rundown on ternary logic over here.
