Centipede not enabled by default on ClusterFuzz

Hi,

According to the [docs](https://google.github.io/oss-fuzz/getting-started/new-project-guide/#fuzzing_engines) and [this line](https://github.com/google/oss-fuzz/blob/b07cc908abf97529a95a4dd9b3d152bf4f101e71/infra/build/functions/build_project.py#L47), Centipede should be enabled by default. Yet, looking at the build logs, it is not built for projects that do not explicitly enable it.

I checked only a few projects, but let's consider the following examples:

**bitcoin-core**
centipede is explicitly [enabled](https://github.com/google/oss-fuzz/blob/b07cc908abf97529a95a4dd9b3d152bf4f101e71/projects/bitcoin-core/project.yaml#L18-L19). The latest build log contains:
```
Starting Step #12 - "compile-centipede-address-x86_64"
...
Starting Step #21 - "compile-centipede-none-x86_64"
```

**abseil-cpp**
No `fuzzing_engines` [defined](https://github.com/google/oss-fuzz/blob/b07cc908abf97529a95a4dd9b3d152bf4f101e71/projects/abseil-cpp/project.yaml) and no centipede in the latest build log.

Start in infra/build/functions/build_project.py at the default fuzzing-engine handling, then compare it with projects/bitcoin-core/project.yaml and projects/abseil-cpp/project.yaml. Check the related build logs to determine why an omitted fuzzing_engines setting does not produce Centipede steps. Done means projects without an explicit setting build Centipede as the documentation describes.

Written by the indexing model from the issue text.