Allow build info properties to be excluded

Update Maven and Gradle plugins to allow build info properties to be
excluded.

Prior to this commit, the `BuildPropertiesWriter` would fail with an
NPE if the group, artifact, name or version properties were `null`.

This was specifically problematic with the Gradle plugin, since its
DSL allows `null` properties which would either be passed to the writer
or, in the case of `artifact`, converted into a string value of
"unspecified".

See gh-27412

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/buildinfo/BuildInfoIntegrationTests-removePropertiesUsingEmptyStrings.gradle

@@ -0,0 +1,16 @@
+plugins {
+	id 'org.springframework.boot' version '{version}' apply false
+}
+
+group = 'foo'
+version = '0.1.0'
+
+task buildInfo(type: org.springframework.boot.gradle.tasks.buildinfo.BuildInfo) {
+	destinationDir project.buildDir
+	properties {
+		group = ''
+		artifact = ''
+		version = ''
+		name = ''
+	}
+}

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/buildinfo/BuildInfoIntegrationTests-removePropertiesUsingNulls.gradle

@@ -0,0 +1,16 @@
+plugins {
+	id 'org.springframework.boot' version '{version}' apply false
+}
+
+group = 'foo'
+version = '0.1.0'
+
+task buildInfo(type: org.springframework.boot.gradle.tasks.buildinfo.BuildInfo) {
+	destinationDir project.buildDir
+	properties {
+		group = null
+		artifact = null
+		version = null
+		name = null
+	}
+}