Correct build script to use empty string as default value (#7)

Resolves #2

Author: OptimumCode

buildSrc/src/main/kotlin/convention.publication.gradle.kts

@@ -21,11 +21,11 @@ if (secretPropsFile.exists()) {
     ext[name.toString()] = value
   }
 } else {
-  ext["signing.keyId"] = System.getenv("SIGNING_KEY_ID")
-  ext["signing.password"] = System.getenv("SIGNING_PASSWORD")
-  ext["signing.keys"] = System.getenv("SIGNING_SECRET_KEY")
-  ext["ossrhUsername"] = System.getenv("OSSRH_USERNAME")
-  ext["ossrhPassword"] = System.getenv("OSSRH_PASSWORD")
+  ext["signing.keyId"] = System.getenv("SIGNING_KEY_ID") ?: ""
+  ext["signing.password"] = System.getenv("SIGNING_PASSWORD") ?: ""
+  ext["signing.keys"] = System.getenv("SIGNING_SECRET_KEY") ?: ""
+  ext["ossrhUsername"] = System.getenv("OSSRH_USERNAME") ?: ""
+  ext["ossrhPassword"] = System.getenv("OSSRH_PASSWORD") ?: ""
 }
 
 val javadocJar by tasks.registering(Jar::class) {