More cautious legacy_color conversion in Logging

While the original line

    color = StyledStrings.Face(foreground=StyledStrings.Legacy.legacy_color(color))

does a perfectly adequate job of mapping old color names to
`SimpleColor`s, it neglects the newer capability of using named `Faces`
with logging.

It is better to first check that the value of `color` can indeed be
recognised as a `legacy_color`-compatible named color, before using it
as the foreground value.

stdlib/Logging/src/ConsoleLogger.jl

@@ -148,7 +148,10 @@ function handle_message(logger::ConsoleLogger, level::LogLevel, message, _module
     # Format lines as text with appropriate indentation and with a box
     # decoration on the left.
     color, prefix, suffix = logger.meta_formatter(level, _module, group, id, filepath, line)::Tuple{Union{Symbol,Int},String,String}
-    color = StyledStrings.Face(foreground=StyledStrings.Legacy.legacy_color(color))
+    lcolor = StyledStrings.Legacy.legacy_color(color)
+    if !isnothing(lcolor)
+        color = StyledStrings.Face(foreground=lcolor)
+    end
     minsuffixpad = 2
     buf = IOBuffer()
     iob = IOContext(buf, stream)