Intro to SQL and Advanced SQL contain exercises with type errors and NA errors

[JohnM-TX]

The following items have errors that prevent successful completion of the exercises, even with the official solution. The errors are one of two variations:

• Intro to SQL, Exercise 5, Q2: operation for a single numeric value tries to act on a list-like object
• Intro to SQL, Exercise 6, Q3: operation fails when encountering an NA value
• Advanced SQL, Exercise 1 Q2: operation fails when encountering an NA value
• Advanced SQL, Exercise 2, Q3: operation fails when encountering an NA value

PR #475