Fix incompatibilities with pre-OAuth2 version of Server Manager - #1820

Enable strictNullChecks and fix a Server Manager compatibility bug

Functional changes

• [src/api/index.ts] / [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.
  • Fall back to the server name when getServerSpec doesn't set name on the returned spec.
• [src/extension.ts]: Expose username and password on ObjectScriptAPI.serverForUri.
• [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.