Remove returning error message when observer-context is null

[NipunaMadhushan]

Purpose

When observability enabled and the project contains global variables declared (or init() functions) with client invokes, the following error occurs in the ballerina runtime.

ballerina: started publishing traces to Jaeger on localhost:55680
ballerina: started Prometheus HTTP listener 0.0.0.0:9797
https://api.asgardeo.io/t/bifrost/oauth2/token/.well-known/openid-configurationerror: client method invocation failed: Span already finished. Can not add tag {http.url:/}

The reason for error is that the spans are created only after declaring variables. Spans won't be created for init() functions as well. Therefore we have to skip returning errors for this scenario.

Also observer context is null only at initializing variables.

Fixes ballerina-platform/ballerina-lang#41966, https://github.com/wso2-enterprise/internal-support-ballerina/issues/556

[warunalakshitha]

LGTM