Fix deprecation compilation warnings

[melbeltagy]

This PR is to fix different compilation warnings due to deprecated code.

• The switch from Spring framework's StringUtils to Apache Commons StringUtils is only applied when the StringUtils.isEmpty is used.
• Switching from URL's constructor to URI.create method is only applied when it's guaranteed to have zero behavioral changes. There are some other instances where behavior changes.

[melbeltagy]

@petkivim , the build keeps failing when it runs on Github.
I ran the command ./gradlew -Dorg.gradle.jvmargs=-Xmx6g --stacktrace clean build test intTest jacocoTestReport -Pfrontend-npm-audit on my local machine against my branch and it is a successful build.

I'm not really sure why it keeps failing on Github though. According to this run, it seems to be failing due to org.niis.xroad.proxy.core.testsuite.ProxyTests but also it passes on my local machine.

Any suggestion how to fix this, please?

[petkivim]

It seems that the last build failure was caused by a timeout while running test with big attachment in ProxyTests. Most likely it was due to the GitHub build environment running slower that usual. Re-runing the build might resolve the issue.