Fix empty signature tooltip after function call (#616)

Fixes #614.

Pulled this from the old LS; it wasn't carried over.

Author: jakebailey

src/LanguageServer/Impl/Sources/SignatureSource.cs

@@ -90,6 +90,10 @@ public async Task<SignatureHelp> GetSignatureAsync(IDocumentAnalysis analysis, S
                                       ?.Item2 ?? -1;
             }
 
+            activeSignature = activeSignature >= 0
+                ? activeSignature
+                : (signatures.Length > 0 ? 0 : -1);
+
             return new SignatureHelp {
                 signatures = signatures.ToArray(),
                 activeSignature = activeSignature,