Test invalid NSString.hexStringFromData values

* Added tests for NSString.hexStringFromData to make sure it can handle nil, NULL, and an empty NSData.

Author: jkasten2

iOS_SDK/OneSignalSDK/UnitTests/UnitTests.m

@@ -2386,4 +2386,10 @@ - (void)testAllowsIncreasedAPNSTokenSize
     XCTAssertEqualObjects(OneSignalClientOverrider.lastHTTPRequest[@"identifier"], UIApplicationOverrider.mockAPNSToken);
 }
 
+- (void)testHexStringFromDataWithInvalidValues {
+    XCTAssertNil([NSString hexStringFromData:nil]);
+    XCTAssertNil([NSString hexStringFromData:NULL]);
+    XCTAssertNil([NSString hexStringFromData:[NSData new]]);
+}
+
 @end