physical: Fix readability-implicit-bool-conversion

```
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../physical.cpp:135:10: error: implicit conversion 'std::basic_string<char>::size_type' (aka 'unsigned long') -> bool [readability-implicit-bool-conversion,-warnings-as-errors]
    if (!color.length())
        ~^
                        == 0u
```

Change-Id: I0bb5e13bec6fbe8b21a89af695ca8f9535e3cdab
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/physical.cpp b/physical.cpp
index fc518f8..aa9d3e4 100644
--- a/physical.cpp
+++ b/physical.cpp
@@ -128,7 +128,7 @@
 {
     static const std::string prefix =
         "xyz.openbmc_project.Led.Physical.Palette.";
-    if (!color.length())
+    if (color.empty())
         return;
     std::string tmp = color;
     tmp[0] = toupper(tmp[0]);