Accessibility: drop-down menus missing placeholder text

#### Describe the task

Some drop-down menus are missing placeholder text, while others are populated, creating an inconsistent user experience.

Add placeholder text to:

Register an operation: Operation info (page 1)
- [ ] Select operation to register for

Submit a report: General stationary combustion excluding line tracing:
- [ ] GSC unit type
- [ ] Fuel name
- [ ] Gas type
- [ ] Methodology

Submit a report: Fuel combustion by mobile equipment
- [ ] Fuel name
- [ ] Gas type
- [ ] Methodology

Start by opening the Register an operation and Submit a report forms named in the issue, then inspect the listed drop-downs alongside populated drop-downs. Add placeholder text to each listed field and verify that the placeholders appear consistently across all three form areas.

Written by the indexing model from the issue text.