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Link to the code that reproduces this issue. A link to a public Github Repository or gist with a minimal reproduction.

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OR
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A step-by-step description of how to reproduce the issue, based on the linked reproduction.

1. Copy code from gist into test/table.ts under describe('getRows')
2. Run test "should return selected fields after consecutive requests"
3. The test fails and an exception is raised

A clear and concise description of what the bug is, and what you expected to happen.

When calling table.getRows more than once, each time with different selectedFields - the method fails with:

TypeError: Cannot read properties of undefined (reading 'mode')
      at <anonymous> (src/bigquery.ts:615:25)
      at Array.map (<anonymous>)
      at mergeSchema (src/bigquery.ts:612:21)
      at Array.map (<anonymous>)
      at Function.mergeSchemaWithRows_ (src/bigquery.ts:609:25)
      at onComplete (src/table.ts:1843:23)
      at <anonymous> (src/table.ts:1891:9)
      at Table.table.request (test/table.ts:2220:11)
      at Table.getRows (src/table.ts:1860:10)
      at Table.wrapper (node_modules/@google-cloud/promisify/build/src/index.js:30:35)
      at Context.<anonymous> (test/table.ts:2222:15)
      at process.processImmediate (node:internal/timers:476:21)
      at process.callbackTrampoline (node:internal/async_hooks:128:17)

It looks like the table object's metadata.schema.fields is affected by the previous call to getRows and becomes an empty array on the second call, causing the method to fail.

A clear and concise description WHY you expect this behavior, i.e., was it a recent change, there is documentation that points to this behavior, etc. **

Calling getRows with valid selectedFields values corresponding to existing columns in the table shouldn't throw an exception.