Handle diagnostics without source

In some cases, compiler diagnostics may not have an associated source,
and for testing whether suggested fixes compile these should be treated
as originating from the same compilation unit of the suggested fix.

Fixes #1873

Fixes #2064

COPYBARA_INTEGRATE_REVIEW=#2064 from schlosna:ds/1873-SuggestedFixes-NPE ddd042b
PiperOrigin-RevId: 350812718

check_api/src/main/java/com/google/errorprone/fixes/SuggestedFixes.java

@@ -1309,8 +1309,10 @@ public CharSequence getCharContent(boolean ignoreEncodingErrors) throws IOExcept
     boolean warningInSameCompilationUnit = false;
     for (Diagnostic<? extends JavaFileObject> diagnostic : diagnosticListener.getDiagnostics()) {
       warningIsError |= diagnostic.getCode().equals("compiler.err.warnings.and.werror");
+      JavaFileObject diagnosticSource = diagnostic.getSource();
+      // If the source's origin is unknown, assume that new diagnostics are due to a modification.
       boolean diagnosticInSameCompilationUnit =
-          diagnostic.getSource().toUri().equals(modifiedFileUri);
+          diagnosticSource == null || diagnosticSource.toUri().equals(modifiedFileUri);
       switch (diagnostic.getKind()) {
         case ERROR:
           ++countErrors;