[release] src/goMain: fix the exception caused by invalid version string

https://go-review.googlesource.com/c/vscode-go/+/289191 made
showGoWelcomePage pass an empty string to shouldShowGoWelcomePage
and causes it to fail. Do the null check on the coerced semver.

Change-Id: Ibc5d9684e153571e7e6507f533367a128952c9ac
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/289969
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>
TryBot-Result: kokoro <[email protected]>
Trust: Hyang-Ah Hana Kim <[email protected]>
Reviewed-by: Suzy Mueller <[email protected]>
(cherry picked from commit feee65b)
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/290109
Reviewed-by: Rebecca Stambler <[email protected]>

Author: hyangah

src/goMain.ts

@@ -603,6 +603,9 @@ export function shouldShowGoWelcomePage(showVersions: string[], newVersion: stri
 	}
 	const coercedNew = semver.coerce(newVersion);
 	const coercedOld = semver.coerce(oldVersion);
+	if (!coercedNew || !coercedOld) {
+		return true;
+	}
 	// Both semver.coerce(0.22.0) and semver.coerce(0.22.0-rc.1) will be 0.22.0.
 	return semver.gte(coercedNew, coercedOld) && showVersions.includes(coercedNew.toString());
 }