[CSV-322] CSVFormat.Builder.setQuote() does not refresh quotedNullString

src/main/java/org/apache/commons/csv/CSVFormat.java

@@ -806,6 +806,8 @@ public Builder setQuote(final Character quoteCharacter) {
                 throw new IllegalArgumentException("The quoteCharacter cannot be a line break");
             }
             this.quoteCharacter = quoteCharacter;
+            final Character quote = quoteCharacter != null ? quoteCharacter : Constants.DOUBLE_QUOTE_CHAR;
+            this.quotedNullString = quote + nullString + quote;
             return this;
         }
 

src/test/java/org/apache/commons/csv/CSVFormatTest.java

@@ -1001,6 +1001,30 @@ void testQuoteCharSameAsDelimiterThrowsException_Deprecated() {
         assertThrows(IllegalArgumentException.class, () -> CSVFormat.DEFAULT.withQuote('!').withDelimiter('!'));
     }
 
+    @Test
+    void testQuotedNullStringTracksQuoteCharacter() throws IOException {
+        final StringBuilder out = new StringBuilder();
+        // @formatter:off
+        final Builder builder = CSVFormat.DEFAULT.builder();
+        final CSVFormat format = builder
+                .setQuoteMode(QuoteMode.ALL)
+                .setNullString("NULL")
+                .get();
+        // @formatter:on
+        format.print(null, out, true);
+        assertEquals("\"NULL\"", out.toString());
+        // set
+        out.setLength(0);
+        builder.setQuote('\'');
+        builder.get().print(null, out, true);
+        assertEquals("'NULL'", out.toString());
+        // reset
+        out.setLength(0);
+        builder.setQuote((Character) null);
+        builder.get().print(null, out, true);
+        assertEquals("\"NULL\"", out.toString());
+    }
+
     @Test
     void testQuoteModeNoneShouldReturnMeaningfulExceptionMessage() {
         final Exception exception = assertThrows(IllegalArgumentException.class, () ->