safely delete screenshots if they are present

[iliyan-velichkov]

to prevent errors like the following:

[ERROR] org.eclipse.dirigible.integration.tests.ui.tests.sample.WebsocketDecoratorSampleProjectIT.testSampleProject -- Time elapsed: 6.064 s <<< ERROR!
java.lang.NullPointerException: Cannot invoke "String.startsWith(String)" because "filePath" is null
        at org.eclipse.dirigible.tests.framework.util.FileUtil.deleteFile(FileUtil.java:120)
        at org.eclipse.dirigible.tests.framework.browser.impl.BrowserImpl.findFirstMatchingElements(BrowserImpl.java:240)
        at org.eclipse.dirigible.tests.framework.browser.impl.BrowserImpl.findElementsInFramesRecursively(BrowserImpl.java:173)
        at org.eclipse.dirigible.tests.framework.browser.impl.BrowserImpl.findOptionalElementInAllFrames(BrowserImpl.java:353)
        at org.eclipse.dirigible.tests.framework.browser.impl.BrowserImpl.findOptionalElementInAllFrames(BrowserImpl.java:336)
        at org.eclipse.dirigible.tests.framework.browser.impl.BrowserImpl.findElementInAllFrames(BrowserImpl.java:149)
        at org.eclipse.dirigible.tests.framework.browser.impl.BrowserImpl.handleElementInAllFrames(BrowserImpl.java:143)
        at org.eclipse.dirigible.tests.framework.browser.impl.BrowserImpl.clickOnElementById(BrowserImpl.java:636)
        at org.eclipse.dirigible.tests.framework.ide.IDE.openGitPerspective(IDE.java:123)
        at org.eclipse.dirigible.integration.tests.ui.tests.sample.SampleProjectRepositoryIT.cloneProject(SampleProjectRepositoryIT.java:46)
        at org.eclipse.dirigible.integration.tests.ui.tests.sample.SampleProjectRepositoryIT.testSampleProject(SampleProjectRepositoryIT.java:31)
        at java.base/java.lang.reflect.Method.invoke(Method.java:580)
        at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
        at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)