[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 <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>
(cherry picked from commit feee65b4)
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/290109


Reviewed-by: Rebecca Stambler <rstambler@golang.org>