src/debugAdapter: remove null items in variables response for maps

We combine the keys and values into a single item in the response for
a variables request, but we left the second slot for each map key-value
in the response as a null item. Filter out the null items to only return
the actual variable.

Fixes golang/vscode-go#199

Change-Id: Iab62dc9ab0ec272ef0042b44abfb68a9e9fdbfc9
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/255782
Trust: Suzy Mueller <suzmue@golang.org>
Run-TryBot: Suzy Mueller <suzmue@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Polina Sokolova <polina@google.com>
diff --git a/src/debugAdapter/goDebug.ts b/src/debugAdapter/goDebug.ts
index 4c2359c..8a51ec9 100644
--- a/src/debugAdapter/goDebug.ts
+++ b/src/debugAdapter/goDebug.ts
@@ -1504,7 +1504,7 @@
 							};
 						});
 					}
-				})
+				}).filter((v) => v != null) // remove the null values created by combining keys and values
 			);
 		} else {
 			variablesPromise = Promise.all(