Entity: do not assume that save IDs are always strings

this is only the first of many changes needed to make entity savedata fully format-agnostic, but it's a start.

tests/phpstan/configs/l7-baseline.neon

@@ -530,6 +530,11 @@ parameters:
 			count: 1
 			path: ../../../src/data/bedrock/LegacyToStringBidirectionalIdMap.php
 
+		-
+			message: "#^Parameter \\#1 \\$value of class pocketmine\\\\nbt\\\\tag\\\\StringTag constructor expects string, string\\|false given\\.$#"
+			count: 1
+			path: ../../../src/entity/EntityFactory.php
+
 		-
 			message: "#^Method pocketmine\\\\entity\\\\EntityFactory\\:\\:getSaveId\\(\\) should return string but returns string\\|false\\.$#"
 			count: 1