[release/6.0] Fix Assembly.LoadFrom resolver

[github-actions]

Backport of #67890 to release/6.0

/cc @agocke @jkotas

Customer Impact

Using Assembly.LoadFrom with either single-file or dynamic assembly (any assembly which doesn't have a Location) can lead to FileLoadException caused by the Assembly.LoadFrom internal implementation.
Note that the LoadFrom can be called on a real assembly file and succeed, but later on trying to load a dependency for an assembly without Location can lead to the exception.

#69596
#67802

Testing

The fix in .NET 7 has been tested for both the single-file and dynamic assembly case manually.

Risk

Low, the code around the fix didn't change between 6 and 7 and 7 didn't see any regression related to this. Also the fix itself is a simple check to avoid calling Path APIs with empty string input.

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Issue Details

---

Backport of #67890 to release/6.0

/cc @agocke @jkotas

Customer Impact

Testing

Risk

IMPORTANT: If this change touches code that ships in a NuGet package, please make certain that you have added any necessary package authoring and gotten it explicitly reviewed.

Author:	github-actions[bot]
Assignees:	-
Labels:	area-AssemblyLoader-coreclr
Milestone:	6.0.x

[jkotas]

Did we hear more people hitting this issue?

[vitek-karas]

@jkotas - yes : #69596

But we should probably fix this regardless because of single-file.
For example:

Assembly.LoadFrom("lib.dll"); // lib.dll exists so this succeeds.
Assembly.Load("NonExistent");

Normally this throws FileNotFoundException, in single-file it throws FileLoadException (with the Path is empty as inner exception).

But even worse (from the bug above):

Assembly.LoadFrom("lib.dll");
Type.GetType("MyType,NonExistent");

Normally the GetType returns null. In single-file it throws FileLoadException. And there's no workaround I could find outside of reimplementing my own LoadFrom.

[vitek-karas]

I updated the issue description to fill in the impact/test/risk sections.

[carlossanlop]

@danmoseley @jeffschwMSFT do we want this change in July servicing? Asking because the Code Complete due date is today and this does not yet have the servicing-approved label.

cc @mmitche in case we need to wait for this change before starting the build. This can be discussed in tomorrow's Tactics.

[agocke]

@carlossanlop This was approved this morning.

[mmitche]

@carlossanlop Please merge when ready.

[danmoseley]

@carlossanlop is out I believe.
I'll merge as the test failure is known (needs a back port of #65280 and its predecessor )