Fix bug in unparser

Fix for #44

Author: levno-710

src/prometheus/unparser.lua

@@ -58,7 +58,7 @@ function Unparser:isValidIdentifier(source)
 	if(string.find(source, self.numberPattern)) then
 		return false;
 	end
-	return true;
+	return #source > 0;
 end
 
 function Unparser:setPrettyPrint(prettyPrint)
@@ -821,4 +821,4 @@ function Unparser:unparseExpression(expression, tabbing)
 	error(string.format("\"%s\" is not a valid unparseable expression", expression.kind));
 end
 
-return Unparser
+return Unparser