LadspaManager: Skip empty entries when looking for plugins

[PhysSong]

Fixes #2569.

[LmmsBot]

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Windows

• Windows 32-bit: lmms-1.2.0.776-mingw-win32.exe (build link)
• Windows 64-bit: lmms-1.2.0.776-mingw-win64.exe (build link)
• Windows 32-bit: lmms-1.2.2-msvc2017-win32.exe (build link)
• Windows 64-bit: lmms-1.2.2-msvc2017-win64.exe (build link)

Linux

• Linux (AppImage): lmms-1.2.0.776-linux-x86_64.AppImage (build link)

🤖

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[Spekular]

Could we pass Qt::SkipEmptyParts as an argument to the split calls that populate ladspaDirectories instead? If there are other places where an empty path could be added, I suppose the check against empty paths needs to be kept.

[PhysSong]

That will also work. I simply copied the logic from PluginFactory because it also does this.

[Spekular]

Then I suppose it's more consistent this way.