Add tests for corner cases

See spring-projectsgh-30290
See spring-projectsgh-30286

Author: sbrannen

spring-core/src/test/java/org/springframework/util/ObjectUtilsTests.java

@@ -847,6 +847,7 @@ void nullSafeConciseToStringForStrings() {
 			String repeat100 = repeat("X", 100);
 			String repeat101 = repeat("X", 101);
 
+			assertThat(ObjectUtils.nullSafeConciseToString("")).isEqualTo("");
 			assertThat(ObjectUtils.nullSafeConciseToString("foo")).isEqualTo("foo");
 			assertThat(ObjectUtils.nullSafeConciseToString(repeat100)).isEqualTo(repeat100);
 			assertThat(ObjectUtils.nullSafeConciseToString(repeat101)).hasSize(truncatedLength).endsWith(truncated);

spring-core/src/test/java/org/springframework/util/StringUtilsTests.java

@@ -799,6 +799,7 @@ void truncatePreconditions() {
 
 	@ParameterizedTest
 	@CsvSource(delimiterString = "-->", value = {
+		"''                  --> ''",
 		"aardvark            --> aardvark",
 		"aardvark12          --> aardvark12",
 		"aardvark123         --> aardvark12 (truncated)...",