Fix file search for empty config type

When config type is unset and filename provided with extension viper failed to properly find that file. 
According to the docs, having set file name with extension is not required to provide config type.

viper_go1_15.go

@@ -35,7 +35,7 @@ func (v *Viper) searchInPath(in string) (filename string) {
 		}
 	}
 
-	if v.configType != "" {
+	if v.configType == "" {
 		if b, _ := exists(v.fs, filepath.Join(in, v.configName)); b {
 			return filepath.Join(in, v.configName)
 		}