Use chars rather than strings

Update `SystemEnvironmentPropertyMapper` to use single chars
rather than strings whenever possible.

See gh-21523

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/source/SystemEnvironmentPropertyMapper.java

@@ -58,7 +58,7 @@ final class SystemEnvironmentPropertyMapper implements PropertyMapper {
 		StringBuilder result = new StringBuilder();
 		for (int i = 0; i < numberOfElements; i++) {
 			if (result.length() > 0) {
-				result.append("_");
+				result.append('_');
 			}
 			result.append(name.getElement(i, Form.UNIFORM).toUpperCase(Locale.ENGLISH));
 		}
@@ -69,7 +69,7 @@ final class SystemEnvironmentPropertyMapper implements PropertyMapper {
 		StringBuilder result = new StringBuilder();
 		for (int i = 0; i < name.getNumberOfElements(); i++) {
 			if (result.length() > 0) {
-				result.append("_");
+				result.append('_');
 			}
 			result.append(convertLegacyNameElement(name.getElement(i, Form.ORIGINAL)));
 		}
@@ -118,7 +118,7 @@ final class SystemEnvironmentPropertyMapper implements PropertyMapper {
 		}
 		StringBuilder legacyCompatibleName = new StringBuilder();
 		for (int i = 0; i < name.getNumberOfElements(); i++) {
-			legacyCompatibleName.append((i != 0) ? "." : "");
+			legacyCompatibleName.append((i != 0) ? '.' : "");
 			legacyCompatibleName.append(name.getElement(i, Form.DASHED).replace('-', '.'));
 		}
 		return ConfigurationPropertyName.isValid(legacyCompatibleName)