How to set/use image URL from user's meta value as WordPress Avatar throughout entire site

functions.php

<?php
add_filter( 'pre_get_avatar_data', 'smyles_set_avatar_based_on_user_meta', 10, 2 );

/**
 * Use URL from User Meta for Avatar
 *
 * This will return a URL value set in the user's meta to use as an avatar, if it
 * exists, and is a valid URL.  Should work everywhere on the site unless you're
 * using a custom avatar plugin that overrides or does not call core get_avatar_data()
 * 
 * This was created to use with my WP Job Manager Field Editor plugin that saves a URL
 * to the user's meta from an upload field, but should work with any site that sets a
 * single URL value in the user's meta.
 *
 * WP Job Manager Field Editor:
 * https://smyl.es/jmfe
 *
 * @requires WordPress 4.2+
 *
 * @author    Myles McNamara
 * @url       https://smyl.es
 * @updated   December 22, 2016
 *
 * @param array $args           Arguments passed to get_avatar_data(), after processing.
 * @param mixed $id_or_email    User ID, User Email, or WP_User/WP_Post/WP_Comment object
 *
 * @return array                Non-null value in `url` key to short circuit get_avatar_data(), or null to continue
 */
function smyles_set_avatar_based_on_user_meta( $args, $id_or_email ){

	// Set this to the full meta key set in Save As under Auto Populate tab (for WP Job Manager Field Editor)
	$user_avatar_meta_key = '_user_avatar';

	// Check for comment_ID
	if( is_object( $id_or_email ) && isset( $id_or_email->comment_ID ) ){
		$id_or_email = get_comment( $id_or_email );
	}

	// Check if WP_Post
	if( $id_or_email instanceof WP_Post ){
		$user_id = $id_or_email->post_author;
	}

	// Check if WP_Comment
	if( $id_or_email instanceof WP_Comment ){
		if( ! empty( $id_or_email->user_id ) ){
			$user_id = $id_or_email->user_id;
		} elseif( ! empty( $id_or_email->comment_author_email ) ){
			// If user_id not available, set as email address to handle below
			$id_or_email = $id_or_email->comment_author_email;
		}
	}

	if( is_numeric( $id_or_email ) ){
		$user_id = $id_or_email;
	} elseif( is_string( $id_or_email ) && strpos( $id_or_email, '@' ) ){
		$id_or_email = get_user_by( 'email', $id_or_email );
	}

	// Last check, convert user object to ID
	if( $id_or_email instanceof WP_User ){
		$user_id = $id_or_email->ID;
	}

	// Now that we have a user ID, check meta for avatar file
	if( ! empty( $user_id ) && is_numeric( $user_id ) ){

		$meta_val = maybe_unserialize( get_user_meta( $user_id, $user_avatar_meta_key, TRUE ) );

		if( ! empty( $meta_val ) ){

			// Set to first value if returned value is array
			if( is_array( $meta_val ) && ! empty( $meta_val[0] ) ){
				$meta_val = $meta_val[0];
			}

			// As long as it's a valid URL, let's go ahead and set it
			if( filter_var( $meta_val, FILTER_VALIDATE_URL ) ){
				$args['url'] = $meta_val;
			}

		}
	}

	return $args;
}