Avoid extra space if condition is empty

Previously, if a builder was created with an empty condition, an extra
space was added before the message. This commit checks for this
particular case and adds a space only when necessary.

Closes gh-8218

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/condition/ConditionMessage.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -302,7 +302,8 @@ public final class ConditionMessage {
 				return new ConditionMessage(ConditionMessage.this, this.condition);
 			}
 			return new ConditionMessage(ConditionMessage.this,
-					this.condition + " " + reason);
+					this.condition + (StringUtils.isEmpty(this.condition) ? "" : " ")
+							+ reason);
 		}
 
 	}

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/condition/ConditionMessageTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -117,6 +117,12 @@ public class ConditionMessageTests {
 		assertThat(message.toString()).isEqualTo("@Test OK");
 	}
 
+	@Test
+	public void forConditionShouldNotAddExtraSpaceWithEmptyCondition() throws Exception {
+		ConditionMessage message = ConditionMessage.forCondition("").because("OK");
+		assertThat(message.toString()).isEqualTo("OK");
+	}
+
 	@Test
 	public void forConditionWhenClassShouldIncludeCondition() throws Exception {
 		ConditionMessage message = ConditionMessage.forCondition(Test.class, "(a=b)")