Allow getCompiler to return None in the test suite

[fjricci]

Summary

Prior to r259433, getCompiler could return None without causing a
test suite crash. However, passing a NoneType to realpath will
cause a crash. Prevent this crash by allowing getCompiler to return None.

[labath]

This doesn't bother me too much, but i'm curious about how are you getting anything reasonable out of the test suite without a working compiler (?)

[fjricci]

I primarily wrote this because getArchFlag() accounts for the possibility that getCompiler() can be None. But my problem was unrelated, so I don't need this (and I agree that it would be surprising if anyone did).

packages/Python/lldbsuite/test/plugins/builder_base.py

def getArchitecture():
     """Returns the architecture in effect the test suite is running with."""
     return os.environ["ARCH"] if "ARCH" in os.environ else ""
 
 
 def getCompiler():
     """Returns the compiler in effect the test suite is running with."""
     compiler = os.environ.get("CC", "clang")
     compiler = lldbutil.which(compiler)
-    return os.path.realpath(compiler)
+    return os.path.realpath(compiler) if compiler else None
 
 
 def getArchFlag():
     """Returns the flag required to specify the arch"""
     compiler = getCompiler()
     if compiler is None:
         return ""
     elif "gcc" in compiler: