[7.1.1] Fix two `bazel mod tidy` crashes (#21700)

* Fixes a crash when using a non-registry override with a specified
version:

```
Caused by: java.lang.NullPointerException: null value in entry: foo=null
	at com.google.common.collect.CollectPreconditions.checkEntryNotNull(CollectPreconditions.java:33)
	at com.google.common.collect.ImmutableMapEntry.<init>(ImmutableMapEntry.java:54)
	at com.google.common.collect.ImmutableMap.entryOf(ImmutableMap.java:345)
	at com.google.common.collect.ImmutableMap$Builder.put(ImmutableMap.java:454)
	at com.google.devtools.build.lib.bazel.bzlmod.Module.getRepoMappingWithBazelDepsOnly(Module.java:67)
	at com.google.devtools.build.lib.bazel.bzlmod.BazelDepGraphFunction.getExtensionUsagesById(BazelDepGraphFunction.java:233)
	at com.google.devtools.build.lib.bazel.bzlmod.BazelDepGraphFunction.compute(BazelDepGraphFunction.java:126)
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:464)
```

* Fixes a crash for root modules with no extension usages:

```
Caused by: java.lang.NullPointerException: Cannot invoke "java.util.Map.keySet()" because the return value of "com.google.common.collect.ImmutableMap.get(Object)" is null
	at com.google.devtools.build.lib.bazel.bzlmod.BazelModTidyFunction.compute(BazelModTidyFunction.java:85)
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:464)
```

Fixes #21651

Closes #21686.

Commit
https://github.com/bazelbuild/bazel/commit/77369dc895597c012794c5ada78705a90a61c2cb



PiperOrigin-RevId: 615826860
Change-Id: I22be3fd53d0dc97aec92afe3dc51a9d6b7e60c98

Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>