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>
1 file changed