[release] src/goInstallTools.ts: avoid type check error

Some versions of go do not seem to print an empty line for
unrecognized env variables.

Change-Id: I7a48dabbe6c0e230fe9a6b3c64944552f8890262
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/245642
Reviewed-by: Brayden Cloud <[email protected]>
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>
(cherry picked from commit 9523e4b)
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/245644
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>

Author: hyangah

src/goInstallTools.ts

@@ -366,7 +366,7 @@ export function updateGoVarsFromConfig(): Promise<void> {
 			if (!process.env['GOBIN'] && envOutput[3] && envOutput[3].trim()) {
 				process.env['GOBIN'] = envOutput[3].trim();
 			}
-			if (!process.env['GOMODCACHE']) {
+			if (!process.env['GOMODCACHE'] && envOutput[4] && envOutput[4].trim()) {
 				process.env['GOMODCACHE'] = envOutput[4].trim();
 			}