Fix TypeError when checking a try/finally block with no catch

[nkeynes]

Currently the detect-unhandled-async-errors rule raises a TypeError when scanning a try block with no catch, e.g.

async function fn() {
   try {
      await foo();
   } finally {
      console.log("done");
   }
}

results in an error:

TypeError: Cannot read properties of null (reading 'type')
Occurred while linting <source-filename>:<line>
Rule: "security-node/detect-unhandled-async-errors"
    at isTryCatchStatement (/Users/nkeynes/src/dylan/node_modules/.pnpm/eslint-plugin-security-node@1.1.4/node_modules/eslint-plugin-security-node/lib/rules/detect-unhandled-async-errors.js:41:21)
    at FunctionDeclaration (/Users/nkeynes/src/dylan/node_modules/.pnpm/eslint-plugin-security-node@1.1.4/node_modules/eslint-plugin-security-node/lib/rules/detect-unhandled-async-errors.js:113:19)
    at ruleErrorHandler (/Users/nkeynes/src/dylan/node_modules/.pnpm/eslint@8.50.0/node_modules/eslint/lib/linter/linter.js:1091:28)
    at /Users/nkeynes/src/dylan/node_modules/.pnpm/eslint@8.50.0/node_modules/eslint/lib/linter/safe-emitter.js:45:58

This PR just adds a check that the handler body exists (and corresponding test).