Fix NULL/False to std::string conversion errors

Fix NULL/False to std::string conversion errors

Newer versions of GCC reject std::string(DE_NULL). Previously this
didn't cause any errors or warnings when compiling, but should produce
an error at runtime. This change replaces invalid values with empty
string literals and adds deFatalStr for cases that potentially
relied on the string constructor throwing an exception.

Components: Framework, OpenGL, Vulkan
VK-GL-CTS issue: 3419

Change-Id: I68ec7d1447d1e96f5d7841e0684687cdd294fe39