fix: version field in technic pack manifest being null

Sometimes, the version field, that is supposed to be a string, was a
null instead. Inspecting other entries, seems like the default for not
having a version should be "", so I made it like that in case the
version was null.

I hope this fixes the issue :^)

launcher/modplatform/technic/SolderPackManifest.cpp

@@ -37,7 +37,7 @@ void loadPack(Pack& v, QJsonObject& obj)
 static void loadPackBuildMod(PackBuildMod& b, QJsonObject& obj)
 {
     b.name = Json::requireString(obj, "name");
-    b.version = Json::requireString(obj, "version");
+    b.version = Json::ensureString(obj, "version", "");
     b.md5 = Json::requireString(obj, "md5");
     b.url = Json::requireString(obj, "url");
 }