Generate a friendly error if a project contains no source files.

Closes #86.

Author: alexrp

src/sdk/build/Vezel.Zig.Sdk.Build.targets

@@ -39,6 +39,9 @@
         <CallTarget Targets="$(_FormatTarget)"
                     Condition="'$(_FormatTarget)' != ''" />
 
+        <Error Text="No source files in project."
+               Condition="@(Compile->Count()) == 0" />
+
         <ZigCompile AccessControl="$(AccessControl)"
                     BlockExtensions="$(BlockExtensions)"
                     CommandFragmentsDirectory="$(_CommandFragmentsPath)"

src/sdk/build/Vezel.Zig.Sdk.Format.targets

@@ -1,14 +1,14 @@
 <Project>
     <Target Name="_CheckFormat"
-            Condition="'$(CompilerMode)' == 'Zig'">
+            Condition="'$(CompilerMode)' == 'Zig' and @(ZigSource->Count()) != 0">
         <ZigFormat FormatterMode="Check"
                    Sources="@(ZigSource)"
                    StandardOutputImportance="high"
                    ToolExe="$(ZigExePath)" />
     </Target>
 
     <Target Name="Format"
-            Condition="'$(CompilerMode)' == 'Zig'">
+            Condition="'$(CompilerMode)' == 'Zig' and @(ZigSource->Count()) != 0">
         <ZigFormat FormatterMode="Execute"
                    Sources="@(ZigSource)"
                    ToolExe="$(ZigExePath)" />