chore: fix sample generation error when schema undefined

[pearigee]

We got the following error in the update-api workflow. We are making some unguarded access (on undefined variables) causing this error to be thrown.

This PR adds guarding to avoid throwing.

2026-07-28T01:46:00.7762143Z Failed to generate API: aiplatform:v1beta1
2026-07-28T01:46:00.7776739Z TypeError: Cannot read properties of undefined (reading 'properties')
2026-07-28T01:46:00.7778968Z     at flattenSchema (/home/runner/work/google-api-nodejs-client/google-api-nodejs-client/google-api-nodejs-client-autodisco/build/src/generator/samplegen.js:117:14)
2026-07-28T01:46:00.7782073Z     at getSample (/home/runner/work/google-api-nodejs-client/google-api-nodejs-client/google-api-nodejs-client-autodisco/build/src/generator/samplegen.js:72:27)
2026-07-28T01:46:00.7785198Z     at addFragments (/home/runner/work/google-api-nodejs-client/google-api-nodejs-client/google-api-nodejs-client-autodisco/build/src/generator/samplegen.js:43:28)
2026-07-28T01:46:00.7788770Z     at async Generator.generate (/home/runner/work/google-api-nodejs-client/google-api-nodejs-client/google-api-nodejs-client-autodisco/build/src/generator/generator.js:224:9)
2026-07-28T01:46:00.7791933Z     at async Generator.generateAPI (/home/runner/work/google-api-nodejs-client/google-api-nodejs-client/google-api-nodejs-client-autodisco/build/src/generator/generator.js:209:24)
2026-07-28T01:46:00.7794939Z     at async /home/runner/work/google-api-nodejs-client/google-api-nodejs-client/google-api-nodejs-client-autodisco/build/src/generator/generator.js:110:17
2026-07-28T01:46:00.7796390Z     at async run (/home/runner/work/google-api-nodejs-client/google-api-nodejs-client/google-api-nodejs-client-autodisco/node_modules/p-queue/dist/index.js:163:29)
2026-07-28T01:46:00.7797159Z aiplatform:v1beta1
2026-07-28T01:46:00.7797368Z -----------
2026-07-28T01:46:00.7797578Z [
2026-07-28T01:46:00.7798118Z   'Attempting first generateAPI call...',
2026-07-28T01:46:00.7798768Z   "GenerateAPI call failed with error: TypeError: Cannot read properties of undefined (reading 'properties'), moving on."
2026-07-28T01:46:00.7799561Z ]
2026-07-28T01:46:00.7799666Z 
2026-07-28T01:55:38.1811154Z Failed to generate API: poly:v1
2026-07-28T01:55:38.1833981Z [Error: ENOENT: no such file or directory, open '/home/runner/work/google-api-nodejs-client/google-api-nodejs-client/google-api-nodejs-client-autodisco/discovery/poly-v1.json'] {
2026-07-28T01:55:38.1835469Z   errno: -2,
2026-07-28T01:55:38.1835807Z   code: 'ENOENT',
2026-07-28T01:55:38.1836146Z   syscall: 'open',
2026-07-28T01:55:38.1837256Z   path: '/home/runner/work/google-api-nodejs-client/google-api-nodejs-client/google-api-nodejs-client-autodisco/discovery/poly-v1.json'
2026-07-28T01:55:38.1838749Z }
2026-07-28T01:55:38.1839077Z poly:v1
2026-07-28T01:55:38.1839367Z -----------
2026-07-28T01:55:38.1839675Z [
2026-07-28T01:55:38.1840083Z   'Attempting first generateAPI call...',
2026-07-28T01:55:38.1841957Z   "GenerateAPI call failed with error: Error: ENOENT: no such file or directory, open '/home/runner/work/google-api-nodejs-client/google-api-nodejs-client/google-api-nodejs-client-autodisco/discovery/poly-v1.json', moving on."
2026-07-28T01:55:38.1843621Z ]