internal/telemetry/event: fix error/value key type tag formatting
internal/telemetry/event: fix error/value key type tag formatting

%v behaves better with nil values.

Change-Id: I4041f5460173c8f420553996d64d9463ef5a0370
Reviewed-on: https://go-review.googlesource.com/c/tools/+/227355
Run-TryBot: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools