Fix formatting a zero period with a weeks unit

See gh-51892

Signed-off-by: Yingzi Zhang <yingzizhang1205@gmail.com>

core/spring-boot/src/main/java/org/springframework/boot/convert/PeriodStyle.java

@@ -31,6 +31,7 @@ import org.springframework.util.Assert;
  *
  * @author Eddú Meléndez
  * @author Edson Chávez
+ * @author Yingzi Zhang
  * @author Valentine Wu
  * @since 2.3.0
  * @see Period
@@ -85,7 +86,7 @@ public enum PeriodStyle {
 		@Override
 		public String print(Period value, @Nullable ChronoUnit unit) {
 			if (value.isZero()) {
-				return Unit.fromChronoUnit(unit).print(value);
+				return "0" + Unit.fromChronoUnit(unit).suffix;
 			}
 			StringBuilder result = new StringBuilder();
 			append(result, value, Unit.YEARS);

core/spring-boot/src/test/java/org/springframework/boot/convert/PeriodStyleTests.java

@@ -29,6 +29,7 @@ import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException
  *
  * @author Eddú Meléndez
  * @author Edson Chávez
+ * @author Yingzi Zhang
  * @author Valentine Wu
  */
 class PeriodStyleTests {
@@ -222,6 +223,11 @@ class PeriodStyleTests {
 		assertThat(PeriodStyle.SIMPLE.print(period, ChronoUnit.YEARS)).isEqualTo("0y");
 	}
 
+	@Test
+	void printSimpleWhenZeroWithWeeksUnitShouldPrintInWeeks() {
+		assertThat(PeriodStyle.SIMPLE.print(Period.ZERO, ChronoUnit.WEEKS)).isEqualTo("0w");
+	}
+
 	@Test
 	void printSimpleWhenNonZeroShouldIgnoreUnit() {
 		Period period = Period.of(1, 2, 3);

core/spring-boot/src/test/java/org/springframework/boot/convert/PeriodToStringConverterTests.java

@@ -32,6 +32,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Eddú Melendez
  * @author Edson Chávez
+ * @author Yingzi Zhang
  */
 class PeriodToStringConverterTests {
 
@@ -63,6 +64,14 @@ class PeriodToStringConverterTests {
 		assertThat(converted).isEqualTo("0y");
 	}
 
+	@ConversionServiceTest
+	void convertWithFormatAndWeeksUnitWhenZeroShouldUseFormatAndUnit(ConversionService conversionService) {
+		String converted = (String) conversionService.convert(Period.ZERO,
+				MockPeriodTypeDescriptor.get(ChronoUnit.WEEKS, PeriodStyle.SIMPLE),
+				TypeDescriptor.valueOf(String.class));
+		assertThat(converted).isEqualTo("0w");
+	}
+
 	@ConversionServiceTest
 	void convertWithFormatAndUnitWhenNonZeroShouldUseFormatAndIgnoreUnit(ConversionService conversionService) {
 		String converted = (String) conversionService.convert(Period.of(1, 0, 3),