fix: apm.startTransaction() crash if agent not yet started

[trentm]

This restores the internal apm._conf as a crutch for
apm.startTransaction() -- and possibly other Agent API methods -- to
function without crashing before apm.start() is called. apm._conf
is set to a config object holding the agent's defaults.

Fixes: #2425

Checklist

• Implement code
• Add tests
• Commit message follows commit guidelines

[trentm]

FWIW because I missed this last time, all the Agent methods that just defer to this._instrumentation.$method(...) (i.e. those like apm.startTransaction()) look to be fine with this change.

[ghost]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2021-11-08T22:01:38.884+0000

• Duration: 20 min 16 sec

• Commit: 6021718

Test stats 🧪

Test	Results
Failed	0
Passed	22
Skipped	0
Total	22

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run module tests for <modules> : Run TAV tests for one or more modules, where <modules> can be either a comma separated list of modules (e.g. memcached,redis) or the string literal ALL to test all modules

• run benchmark tests : Run the benchmark test only.