ICU-22838 Address feedback from aheninger

* icu4c/source/i18n/decimfmt.cpp: Explicitly set the pointers to nullptr
  after deleting them.
* icu4c/source/i18n/number_mapper.h: Use `= {}` to initialize pointers
  to nullptr instead of `= nullptr` to keep project-wide consistency.

icu4c/source/i18n/decimfmt.cpp

@@ -1647,7 +1647,9 @@ void DecimalFormat::touch(UErrorCode& status) {
     delete fields->atomicCurrencyParser.exchange(nullptr);
 #else
     delete fields->atomicParser;
+    fields->atomicParser = nullptr;
     delete fields->atomicCurrencyParser;
+    fields->atomicCurrencyParser = nullptr;
 #endif
 
     // In order for the getters to work, we need to populate some fields in NumberFormat.

icu4c/source/i18n/number_mapper.h

@@ -201,7 +201,7 @@ struct DecimalFormatFields : public UMemory {
 #if U_HAVE_ATOMICS
     std::atomic<::icu::numparse::impl::NumberParserImpl*> atomicParser = {};
 #else
-    ::icu::numparse::impl::NumberParserImpl* atomicParser = nullptr;
+    ::icu::numparse::impl::NumberParserImpl* atomicParser = {};
 #endif
 
     /** The lazy-computed parser for .parseCurrency() */