Fix bug in user import-csv STDIN detection

Author: schlessera

src/WP_CLI/Entity/Utils.php

@@ -10,13 +10,25 @@ class Utils {
 	 * @return bool
 	 */
 	public static function has_stdin() {
-		$handle = fopen( 'php://stdin', 'r' );
-		$read   = array( $handle );
-		$write  = null;
-		$except = null;
+		$handle  = fopen( 'php://stdin', 'r' );
+		$read    = array( $handle );
+		$write   = null;
+		$except  = null;
 		$streams = stream_select( $read, $write, $except, 0 );
+		$fstat   = fstat( $handle );
+
 		fclose( $handle );
 
-		return 1 === $streams;
+		if ( $streams !== 1 ) {
+			// No stream on STDIN.
+			return false;
+		}
+
+		if ( isset( $fstat['size'] ) && $fstat['size'] === 0 ) {
+			// A stream on STDIN was detected but has a size of 0 bytes.
+			return false;
+		}
+
+		return true;
 	}
 }