Mark inputFilePath as optional in findConfiguration()

When `suppliedConfigFilePath` is passed to `findConfigurationPath()`, `inputFilePath` is completely ignored and should therefore be optional

src/configuration.ts

@@ -90,7 +90,7 @@ const BUILT_IN_CONFIG = /^tslint:(.*)$/;
  * of the search for a configuration.
  * @returns Load status for a TSLint configuration object
  */
-export function findConfiguration(configFile: string | null, inputFilePath: string): IConfigurationLoadResult {
+export function findConfiguration(configFile: string | null, inputFilePath?: string): IConfigurationLoadResult {
     const configPath = findConfigurationPath(configFile, inputFilePath);
     const loadResult: IConfigurationLoadResult = { path: configPath };
 
@@ -112,14 +112,14 @@ export function findConfiguration(configFile: string | null, inputFilePath: stri
  * @returns An absolute path to a tslint.json file
  * or undefined if neither can be found.
  */
-export function findConfigurationPath(suppliedConfigFilePath: string | null, inputFilePath: string) {
+export function findConfigurationPath(suppliedConfigFilePath: string | null, inputFilePath?: string) {
     if (suppliedConfigFilePath != null) {
         if (!fs.existsSync(suppliedConfigFilePath)) {
             throw new FatalError(`Could not find config file at: ${path.resolve(suppliedConfigFilePath)}`);
         } else {
             return path.resolve(suppliedConfigFilePath);
         }
-    } else {
+    } else if (inputFilePath) {
         // convert to dir if it's a file or doesn't exist
         let useDirName = false;
         try {
@@ -149,10 +149,9 @@ export function findConfigurationPath(suppliedConfigFilePath: string | null, inp
                 return path.resolve(configFilePath);
             }
         }
-
-        // no path could be found
-        return undefined;
     }
+    // no path could be found
+    return undefined;
 }
 
 /**