GRF: Add an error check for empty RSM node names

Some of them are just the empty string. Decoding them throws with an iconv error ("Invalid argument"). Shouldn't affect cdata.

Core/FileFormats/RagnarokGRF.lua

@@ -184,6 +184,10 @@ function RagnarokGRF:DecodeFileEntries()
 end
 
 function RagnarokGRF:DecodeFileName(input)
+	if input == "" then
+		return input
+	end
+
 	-- This should likely be moved since it won't happen at decoding time, only on demand (in other decoders)
 	if type(input) == "string" then
 		local unicodeFilePath, err = iconv.convert(input, "CP949", "UTF-8")

Tests/FileFormats/RagnarokGRF.spec.lua

@@ -410,5 +410,9 @@ describe("RagnarokGRF", function()
 			local pointerToNullTerminatedStringBytes = inputBuffer:ref()
 			assertEquals(RagnarokGRF:DecodeFileName(pointerToNullTerminatedStringBytes), "hello/world.txt")
 		end)
+
+		it("should return empty strings unchanged", function()
+			assertEquals(RagnarokGRF:DecodeFileName(""), "")
+		end)
 	end)
 end)