Fix timeline for radio/custom when correct answer isn't given

Fix #41306

Test plan: Check that each of the following now do the right thing:
http://localhost:8000/exercises/recognizing_rays_lines_and_line_segments.html?debug&problem=line-segment&seed=193&activity=%5B%5B%22incorrect-activity%22%2C%22%5C%22Ray%5C%22%22%2C5%5D%2C%5B%22incorrect-activity%22%2C%22%5C%22Line%5C%22%22%2C1%5D%2C%5B%22correct-activity%22%2C%22%5C%22Line%20segment%5C%22%22%2C2%5D%5D

http://localhost:8000/exercises/recognizing_rays_lines_and_line_segments.html?debug&problem=line-segment&seed=193&activity=%5B%5B%22incorrect-activity%22%2C%22%5C%22Ray%5C%22%22%2C8%5D%2C%5B%22incorrect-activity%22%2C%22%5C%22Line%5C%22%22%2C1%5D%5D

http://localhost:8000/exercises/balancing_chemical_equations.html?debug&problem=mgoh2-hcl---mgcl2-h2o&seed=32&activity=%5B%5B%22incorrect-activity%22%2C%22%5B%5C%222%5C%22%2C%5C%22%5C%22%2C%5C%222%5C%22%2C%5C%22%5C%22%5D%22%2C6%5D%2C%5B%22incorrect-activity%22%2C%22%5B%5C%222%5C%22%2C%5C%222%5C%22%2C%5C%222%5C%22%2C%5C%22%5C%22%5D%22%2C3%5D%2C%5B%22incorrect-activity%22%2C%22%5B%5C%222%5C%22%2C%5C%22%5C%22%2C%5C%222%5C%22%2C%5C%22%5C%22%5D%22%2C10%5D%2C%5B%22correct-activity%22%2C%22%5B%5C%22%5C%22%2C%5C%222%5C%22%2C%5C%22%5C%22%2C%5C%222%5C%22%5D%22%2C6%5D%5D

http://localhost:8000/exercises/balancing_chemical_equations.html?debug&problem=mgoh2-hcl---mgcl2-h2o&seed=32&activity=%5B%5B%22incorrect-activity%22%2C%22%5B%5C%222%5C%22%2C%5C%22%5C%22%2C%5C%222%5C%22%2C%5C%22%5C%22%5D%22%2C3%5D%2C%5B%22incorrect-activity%22%2C%22%5B%5C%224%5C%22%2C%5C%222%5C%22%2C%5C%224%5C%22%2C%5C%22%5C%22%5D%22%2C8%5D%5D

Auditors: cbhl, alpert