Fix empty signature tooltip after function call (microsoft#616)

Fixes microsoft#614.

Author: AlexanderSher

src/LanguageServer/Impl/Sources/SignatureSource.cs

@@ -84,12 +84,16 @@ public async Task<SignatureHelp> GetSignatureAsync(IDocumentAnalysis analysis, S
             if (activeParameter >= 0) {
                 // TODO: Better selection of active signature by argument set
                 activeSignature = signatures
-                                      .Select((s, i) => Tuple.Create(s, i))
-                                      .OrderBy(t => t.Item1.parameters.Length)
-                                      .FirstOrDefault(t => t.Item1.parameters.Length > activeParameter)
-                                      ?.Item2 ?? -1;
+                    .Select((s, i) => Tuple.Create(s, i))
+                    .OrderBy(t => t.Item1.parameters.Length)
+                    .FirstOrDefault(t => t.Item1.parameters.Length > activeParameter)
+                    ?.Item2 ?? -1;
             }
 
+            activeSignature = activeSignature >= 0
+                ? activeSignature
+                : (signatures.Length > 0 ? 0 : -1);
+
             return new SignatureHelp {
                 signatures = signatures.ToArray(),
                 activeSignature = activeSignature,