Projucer: Default to "Sign to Run Locally" when no signing identity is specified

By default, x86_64 binaries are not signed, but arm64 binaries are. When
building universal binaries, it was possible to end up with a final
artefact that contains an unsigned x86_64 image, but a signed arm64
image. In some cases, this could cause issues in subsequent codesign
invocations.

extras/Projucer/Source/ProjectSaving/jucer_ProjectExport_Xcode.h

@@ -2466,7 +2466,8 @@ private:
         if (isUsingDefaultSigningIdentity (config))
             return iOS ? "iPhone Developer" : "Mac Developer";
 
-        return config.getCodeSignIdentityString();
+        const auto identity = config.getCodeSignIdentityString();
+        return identity.isNotEmpty() ? identity : "-";
     }
 
     StringPairArray getProjectSettings (const XcodeBuildConfiguration& config) const