Rename _NULL to _NO_ENTITY (#3382)

* Remame `_NULL` to `_NO_ENTITY`

A more understandable name because some people might not understand the NULL concept

* Woops

Author: Astralcircle

lua/entities/gmod_wire_expression2/core/entity.lua

@@ -91,9 +91,9 @@ end
 --[[******************************************************************************]]
 -- Functions getting string
 
-E2Lib.registerConstant("NULL", NULL)
+E2Lib.registerConstant("NO_ENTITY", NULL, "An invalid entity")
 
-[deprecated = "Use the constant NULL instead"]
+[deprecated = "Use the constant NO_ENTITY instead"]
 e2function entity noentity()
 	return NULL
 end