Bug: blank json triggers a cryptic error message and a 500 server error on Activity pages

#### Describe the Bug:

Submitting an activity page without data triggers a server error instead of either being accepted by the system, or showing a form validation error.

Question for BA: is it allowed to select an activity, then report no emissions on it?

#### Probability:

(How likely the bug is to happen, scored from 1-5. For example, probability of 5 is something like "it happens to all users every time they log in." whereas probability of 1 "only happens to certain users when a really specific and unlikely path is followed.")

#### Impact:

(How bad the bug is when it does happen, scored from 1-5. For example, effect of 5 is "the entire app crashes and makes it unusable for all users" or "the bug causes the wrong data to be saved, with critical information (e.g. payment) being affected." whereas effect of 1 is "It makes some styling look a little bit weird.")

#### Steps to reproduce the behaviour:

1. Step #1
2. Step #2
3. Step #3

#### Screenshots:

#### Additional information:

OS: [e.g. iOS]
Browser: [e.g. chrome, safari]
Version: [e.g. 22]

#### Additional Context

-

No file, test, or entry point is named. Reproduce submitting an Activity page with blank JSON, then trace the Activity page submission and error handling; confirm with the BA whether no emissions is valid, and consider the issue done when the chosen behavior avoids the cryptic message and 500 response.

Written by the indexing model from the issue text.