ICU-22671 format_date should use c++ nullptr instead of 0 for udat_open/DateFormat::create

- [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22671
- [x] Required: The PR title must be prefixed with a JIRA Issue number. <!-- For example: "ICU-1234 Fix xyz" -->
- [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
- [x] Required: Each commit message must be prefixed with a JIRA Issue number. <!-- For example: "ICU-1234 Fix xyz" -->
- [ ] Issue accepted (done by Technical Committee after discussion)
- [ ] Tests included, if applicable
- [ ] API docs and/or User Guide docs changed or added, if applicable

icu4c/source/i18n/datefmt.cpp

@@ -561,7 +561,7 @@ DateFormat::create(EStyle timeStyle, EStyle dateStyle, const Locale& locale)
     // This should never really happen, because the preceding constructor
     // should always succeed.  If the resource data is unavailable, a last
     // resort object should be returned.
-    return 0;
+    return nullptr;
 }
 
 //----------------------------------------------------------------------

icu4c/source/i18n/udat.cpp

@@ -142,7 +142,7 @@ udat_open(UDateFormatStyle  timeStyle,
 {
     DateFormat *fmt;
     if(U_FAILURE(*status)) {
-        return 0;
+        return nullptr;
     }
     if(gOpener!=nullptr) { // if it's registered
       fmt = (DateFormat*) (*gOpener)(timeStyle,dateStyle,locale,tzID,tzIDLength,pattern,patternLength,status);
@@ -186,7 +186,7 @@ udat_open(UDateFormatStyle  timeStyle,
         if(zone == 0) {
             *status = U_MEMORY_ALLOCATION_ERROR;
             delete fmt;
-            return 0;
+            return nullptr;
         }
         fmt->adoptTimeZone(zone);
     }