Now proxy is not set when http.proxyHost or http.proxyPort are blank instead of null

Currently there is a bug due to checking if proxy related system properties are null. In some cases (for example when launching from IntelliJ) those properties can be empty strings instead of null:
12:04:16.515 [DEBUG] [org.gradle.api.internal.artifacts.ivyservice.IvyLoggingAdaper] setting 'http.proxyPort' to ''