Avoid a possible IndexOutOfBoundsException in MemberName.

Ordinarily we shouldn't see an empty class name but it appears that sometimes we do: []

```
Sep 26, 2024 10:29:02 AM com.google.devtools.staticanalysis.pipeline.java.errorprone.ErrorProneFindingsJavacAnalyzer$1 handleError
WARNING: crash in Error Prone analyzer MemberName [CONTEXT ratelimit_period="5 SECONDS" analyzer="JavaStyle" ]
java.lang.StringIndexOutOfBoundsException: Index 0 out of bounds for length 0
	at java.base/jdk.internal.util.Preconditions$1.apply(Preconditions.java:55)
	at java.base/jdk.internal.util.Preconditions$1.apply(Preconditions.java:52)
	at java.base/jdk.internal.util.Preconditions$4.apply(Preconditions.java:213)
	at java.base/jdk.internal.util.Preconditions$4.apply(Preconditions.java:210)
	at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:98)
	at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:106)
	at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:302)
	at java.base/java.lang.String.checkIndex(String.java:4881)
	at java.base/java.lang.StringLatin1.charAt(StringLatin1.java:46)
	at java.base/java.lang.String.charAt(String.java:1582)
	at com.google.errorprone.bugpatterns.MemberName.isConformantUpperCamelName(MemberName.java:290)
	at com.google.errorprone.bugpatterns.MemberName.matchClass(MemberName.java:111)
	at com.google.errorprone.scanner.ErrorProneScanner.processMatchers(ErrorProneScanner.java:460)
	at com.google.errorprone.scanner.ErrorProneScanner.visitClass(ErrorProneScanner.java:558)
	at com.google.errorprone.scanner.ErrorProneScanner.visitClass(ErrorProneScanner.java:157)
```
PiperOrigin-RevId: 679249176

core/src/main/java/com/google/errorprone/bugpatterns/MemberName.java

@@ -101,7 +101,8 @@ public final class MemberName extends BugChecker
   public Description matchClass(ClassTree tree, VisitorState state) {
     ClassSymbol symbol = getSymbol(tree);
     String name = tree.getSimpleName().toString();
-    if (isConformantUpperCamelName(name)) {
+    if (name.isEmpty() || isConformantUpperCamelName(name)) {
+      // name.isEmpty() should not happen normally but could if there are errors.
       return NO_MATCH;
     }
     String renamed = suggestedClassRename(name);