Accept zero length queries

tests/Query.cpp

@@ -0,0 +1,20 @@
+#include <iostream>
+#include <cassert>
+
+#include "../src/QueryParser.h"
+
+int main() {
+
+    {
+        std::string buf = "?test1=&test2=someValue";
+        assert(uWS::getDecodedQueryValue("test2", (char *) buf.data()) == "someValue");
+    }
+
+    {
+        std::string buf = "?test1=&test2=someValue";
+        assert(uWS::getDecodedQueryValue("test1", (char *) buf.data()) == "");
+        assert(uWS::getDecodedQueryValue("test2", (char *) buf.data()) == "someValue");
+    }
+
+    return 0;
+}