Fix default display data

[blairlyons]

Problem

Agents that have 0 subpoints get assigned a default display_type of SPHERE_GROUP rather than SPHERE.

Solution

Added a check for 0 subpoints in AgentData._default_display_type_for_agent().

Type of change

• Bug fix (non-breaking change which fixes an issue)

Change summary:

• only set default DISPLAY_TYPE to something other than SPHERE if n_subpoints > 0
• added a test case where a sphere agent doesn't have display data specified so uses the default

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 :tada:

Comparison is base (455f3ca) 92.08% compared to head (0a817c0) 92.12%.

:mega: This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #144 +/- ## ========================================== + Coverage 92.08% 92.12% +0.04% ========================================== Files 101 101 Lines 4394 4396 +2 ========================================== + Hits 4046 4050 +4 + Misses 348 346 -2 ``` | [Impacted Files](https://app.codecov.io/gh/simularium/simulariumio/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [simulariumio/tests/conftest.py](https://app.codecov.io/gh/simularium/simulariumio/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c2ltdWxhcml1bWlvL3Rlc3RzL2NvbmZ0ZXN0LnB5) | `100.00% <ø> (ø)` | | | [simulariumio/data\_objects/agent\_data.py](https://app.codecov.io/gh/simularium/simulariumio/pull/144?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c2ltdWxhcml1bWlvL2RhdGFfb2JqZWN0cy9hZ2VudF9kYXRhLnB5) | `90.07% <100.00%> (+0.84%)` | :arrow_up: |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.