Replace unnecessary Preconditions by assert

In many places Preconditions are redundant and have no any sense. Many Preconditions are running in production and are checking something that has no any sense. I am suggestion to replace **unnecessary** `Preconditions` by `assert` because `assert` is disabled by default and is commonly used only for testing purposes or hints in code.
By using `assert` users will be able to `enable` those checks if they think they are needed, otherwise unnecessary checks will be disabled by default.

For example, place with the following check has no sense as we are throwing `NPE` when executing `additions.isEmpty()`, so `checkNotNull` is redundant.
```
Preconditions.checkNotNull(additions);
if (additions.isEmpty()){
additions=null;
}
```
Thus, we can simply replace it with the following code:
```
assert additions!=null;
if (additions.isEmpty()){
additions=null;
}
```

An example of needed checks might something like:
```
this.additions = Preconditions.checkNotNull(additions);
```

No files, tests, or entry points are named. Start by inventorying the repository's Preconditions uses and distinguishing redundant checks from checks whose values are assigned or otherwise required. Done means the agreed unnecessary checks are replaced without changing required validation behavior, with relevant tests passing.

Written by the indexing model from the issue text.