Remove erroneous check about a repo mapping entry with an empty apparent name

It turns out that not only the main repo can have such an entry; any repo generated by a module extension hosted in the main repo can (and will) also have such an entry, because they can see all repos that the main repo can see, which includes the main repo itself.

PiperOrigin-RevId: 483938753
Change-Id: I6192496f2edf08c9b1723c5b041c0ac44f928b3e

Author: Wyverald

src/main/java/com/google/devtools/build/lib/analysis/RepoMappingManifestAction.java

@@ -17,7 +17,6 @@
 import static java.util.Comparator.comparing;
 
 import com.google.auto.value.AutoValue;
-import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.devtools.build.lib.actions.ActionExecutionContext;
 import com.google.devtools.build.lib.actions.ActionKeyContext;
@@ -106,9 +105,6 @@ public DeterministicWriter newDeterministicWriter(ActionExecutionContext ctx)
         if (entry.targetRepoApparentName().isEmpty()) {
           // The apparent repo name can only be empty for the main repo. We skip this line as
           // Rlocation paths can't reference an empty apparent name anyway.
-          Preconditions.checkArgument(
-              entry.sourceRepo().isMain(),
-              "only the main repo mapping can contain an entry with an empty apparent name");
           continue;
         }
         // The canonical name of the main repo is the empty string, which is not a valid name for a