fix(dataproxy): no trailing slash if proxypath is empty

The JoinUrlFragments function adds a trailing slash if to the proxy url
if the proxy path is an empty string. This fix removes that trailing
slash. Fixes #3847

pkg/util/url.go

@@ -27,6 +27,11 @@ func (r *UrlQueryReader) Get(name string, def string) string {
 func JoinUrlFragments(a, b string) string {
 	aslash := strings.HasSuffix(a, "/")
 	bslash := strings.HasPrefix(b, "/")
+
+	if len(b) == 0 {
+		return a
+	}
+
 	switch {
 	case aslash && bslash:
 		return a + b[1:]