Fix incompatibilities with pre-OAuth2 version of Server Manager

[isc-klu]

Enable strictNullChecks and fix a Server Manager compatibility bug

Functional changes

• [client/src/extension.ts]: Patch ServerManagerAPI for compatibility with pre-OAuth2 versions of Server Manager: Construct a BasicAuthorization when Server Manager doesn't supply an auth object.
• [tsconfig.base.json]: Enable strictNullChecks, so mistakes like the ones above get caught by the compiler instead of at runtime.

Non-functional changes

Everything else is mechanical fallout from enabling strictNullChecks, limited to:

• Adding a missing type annotation.
• Loosening a type annotation to include undefined/null where that reflects actual runtime behavior.
• Adding non-null assertions (!) to values of which the type is nullable.

No behavior changes outside the items listed above.