Fix conflict with ObjectiveC nil

Author: Kyle-Ye

Sources/OpenGraph_SPI/Attribute/AttributeID.hpp

@@ -22,7 +22,7 @@ class AttributeID final {
     enum class Kind: uint8_t {
         Direct = 0x0,
         Indirect = 0x1,
-        Nil = 0x2,
+        Null = 0x2,
     };
 private:
     uint32_t _rawValue;
@@ -59,8 +59,8 @@ class AttributeID final {
     }
 
     OG_INLINE OG_CONSTEXPR
-    const bool isNil() const OG_NOEXCEPT {
-        return getKind() == Kind::Nil;
+    const bool isNull() const OG_NOEXCEPT {
+        return getKind() == Kind::Null;
     }
 
     OG_INLINE OG_CONSTEXPR

Sources/OpenGraph_SPI/Attribute/OGAttribute.cpp

@@ -11,7 +11,7 @@
 #include "../Util/assert.hpp"
 #include <optional>
 
-const OGAttribute OGAttributeNil = OGAttribute(OG::AttributeID::Kind::Nil);
+const OGAttribute OGAttributeNil = OGAttribute(OG::AttributeID::Kind::Null);
 
 OGAttribute OGGraphGetCurrentAttribute() {
     // TODO