Use std::optional for heart rate

[WhyNotHugo]

If no value is available, use null. Use this distinction to differentiate in the UI between missing data and zero value.

Still sends value 0 over BLE. I'm unsure whether this should change or not

[github-actions[bot]]

Build size and comparison to main:

Section	Size	Difference
text	379348B	144B
data	944B	0B
bss	22536B	0B

Run in InfiniEmu

[WhyNotHugo]

Checks have not completed because they require maintainer approval.

This isn't something I can do anything about, I'm not sure what's the purpose of this warning.

[WhyNotHugo]

Building the simulator fails.

docker run --rm -it -v ${PWD}:/sources infinitime-build doesn't build the simulator too. I tried tweaking it to build it, but apparently multiple other dependencies are missing.

Is there an existing setup for this which I can use?

[WhyNotHugo]

See: https://github.com/InfiniTimeOrg/InfiniSim/pull/180