Fix UI warning about NaNs (#4142)

`e.target.valueAsNumber` converts empty strings to NaNs, but passing
them to value attribute doesn't convert them back to empty strings.

```
## Error Type
Console Error

## Error Message
Received NaN for the `value` attribute. If this is expected, cast the value to a string.


    at input (<anonymous>:null:null)
    at TextField (lib/TextField/TextField.tsx:25:5)
    at EditMirror (app/mirrors/[mirrorId]/edit/page.tsx:207:13)

## Code Frame
  23 |   }
  24 |   return (
> 25 |     <BaseInputField
     |     ^
  26 |       {...(inputProps as ComponentProps<typeof BaseInputField>)}
  27 |     />
  28 |   );

Next.js version: 16.1.6 (Webpack)
```

Author: ilidemi

ui/app/mirrors/[mirrorId]/edit/page.tsx

@@ -193,7 +193,7 @@ export default function EditMirror({ params }: EditMirrorProps) {
                   batchSize: e.target.valueAsNumber,
                 })
               }
-              value={config.batchSize}
+              value={Number.isNaN(config.batchSize) ? '' : config.batchSize}
             />
           </div>
         }
@@ -213,7 +213,7 @@ export default function EditMirror({ params }: EditMirrorProps) {
                   idleTimeout: e.target.valueAsNumber,
                 })
               }
-              value={config.idleTimeout}
+              value={Number.isNaN(config.idleTimeout) ? '' : config.idleTimeout}
             />
           </div>
         }
@@ -233,7 +233,11 @@ export default function EditMirror({ params }: EditMirrorProps) {
                   snapshotNumRowsPerPartition: e.target.valueAsNumber,
                 })
               }
-              value={config.snapshotNumRowsPerPartition}
+              value={
+                Number.isNaN(config.snapshotNumRowsPerPartition)
+                  ? ''
+                  : config.snapshotNumRowsPerPartition
+              }
             />
           </div>
         }
@@ -253,7 +257,11 @@ export default function EditMirror({ params }: EditMirrorProps) {
                   snapshotMaxParallelWorkers: e.target.valueAsNumber,
                 })
               }
-              value={config.snapshotMaxParallelWorkers}
+              value={
+                Number.isNaN(config.snapshotMaxParallelWorkers)
+                  ? ''
+                  : config.snapshotMaxParallelWorkers
+              }
             />
           </div>
         }
@@ -273,7 +281,11 @@ export default function EditMirror({ params }: EditMirrorProps) {
                   snapshotNumTablesInParallel: e.target.valueAsNumber,
                 })
               }
-              value={config.snapshotNumTablesInParallel}
+              value={
+                Number.isNaN(config.snapshotNumTablesInParallel)
+                  ? ''
+                  : config.snapshotNumTablesInParallel
+              }
             />
           </div>
         }