Improve the error message when additional build-info prop has null value

Closes gh-6724

spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/BuildPropertiesWriter.java

@@ -22,6 +22,7 @@ import java.io.IOException;
 import java.text.SimpleDateFormat;
 import java.util.Date;
 import java.util.Map;
+import java.util.Map.Entry;
 import java.util.Properties;
 
 /**
@@ -119,9 +120,21 @@ public final class BuildPropertiesWriter {
 			this.artifact = artifact;
 			this.name = name;
 			this.version = version;
+			validateAdditionalProperties(additionalProperties);
 			this.additionalProperties = additionalProperties;
 		}
 
+		private static void validateAdditionalProperties(
+				Map<String, String> additionalProperties) {
+			if (additionalProperties != null) {
+				for (Entry<String, String> property : additionalProperties.entrySet()) {
+					if (property.getValue() == null) {
+						throw new NullAdditionalPropertyValueException(property.getKey());
+					}
+				}
+			}
+		}
+
 		public String getGroup() {
 			return this.group;
 		}
@@ -143,4 +156,16 @@ public final class BuildPropertiesWriter {
 		}
 
 	}
+
+	/**
+	 * Exception thrown when an additional property with a null value is encountered.
+	 */
+	public static class NullAdditionalPropertyValueException
+			extends IllegalArgumentException {
+
+		public NullAdditionalPropertyValueException(String name) {
+			super("Additional property '" + name + "' is illegal as its value is null");
+		}
+
+	}
 }