Add tests for corner cases

See gh-30290
See gh-30286

Author: sbrannen

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

@@ -821,6 +821,7 @@ void nullSafeConciseToStringForStrings() {
 			String repeat100 = "X".repeat(100);
 			String repeat101 = "X".repeat(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

@@ -771,6 +771,7 @@ void truncatePreconditions() {
 
 	@ParameterizedTest
 	@CsvSource(delimiterString = "-->", textBlock = """
+			''                  --> ''
 			aardvark            --> aardvark
 			aardvark12          --> aardvark12
 			aardvark123         --> aardvark12 (truncated)...