Update functions.php to fix deprecated warnings

Fixed two pieces of code that were generating deprecated warnings:
-- Backtrace from warning 'strpos(): Passing null to parameter WordPress#1 ($haystack) of type string is deprecated' at /wp-includes/functions.php 7329:
this was the code causing the error: $scheme_separator = strpos( $path, '://' );

-- Backtrace from warning 'str_replace(): Passing null to parameter WordPress#3 ($subject) of type array|string is deprecated' at /wp-includes/functions.php 2189:
this was the code causing the error: $path = str_replace( '\\', '/', $path );

Author: RobertP3469

src/wp-includes/functions.php

@@ -2192,7 +2192,7 @@ function wp_normalize_path( $path ) {
 	}
 
 	// Standardize all paths to use '/'.
-	$path = str_replace( '\\', '/', $path );
+	$path = str_replace('\\', '/', $path ?? '');
 
 	// Replace multiple slashes down to a singular, allowing for network shares having two slashes.
 	$path = preg_replace( '|(?<=.)/+|', '/', $path );
@@ -7357,7 +7357,7 @@ function _device_can_upload() {
  * @return bool True if the path is a stream URL.
  */
 function wp_is_stream( $path ) {
-	$scheme_separator = strpos( $path, '://' );
+	$scheme_separator = strpos($path ?? '', '://');
 
 	if ( false === $scheme_separator ) {
 		// $path isn't a stream.