Simple fix for #4721

This makes relative outDir and other file path options not ignored when they point to the working directory.
No tests are failed.

Another way to fix this is to use `options.somePathOprtion !== undefined` check everywhere, but it would be a more complex change.
What do you think?

src/compiler/commandLineParser.ts

@@ -446,6 +446,9 @@ namespace ts {
                             }
                             if (opt.isFilePath) {
                                 value = normalizePath(combinePaths(basePath, value));
+                                if (value === "") {
+                                    value = ".";
+                                }
                             }
                             options[opt.name] = value;
                         }
@@ -495,4 +498,4 @@ namespace ts {
             return fileNames;
         }
     }
-}
+}