[flutter_goldens] Remove dead check on null being in a list of non-nullables (flutter#183938)

To complete dart-lang/sdk#57101, we must
remove these checks which would be reported by the analyzer. These two
lists cannot contain `null`.

Work towards dart-lang/sdk#57101

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [ ] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

Author: srawlins

packages/flutter_goldens/lib/skia_client.dart

@@ -177,15 +177,6 @@ class SkiaGoldClient {
       '--passfail',
     ];
 
-    if (imgtestInitCommand.contains(null)) {
-      final buf = StringBuffer()
-        ..writeln('A null argument was provided for Skia Gold imgtest init.')
-        ..writeln('Please confirm the settings of your golden file test.')
-        ..writeln('Arguments provided:');
-      imgtestInitCommand.forEach(buf.writeln);
-      throw SkiaException(buf.toString());
-    }
-
     final io.ProcessResult result = await process.run(imgtestInitCommand);
 
     if (result.exitCode != 0) {
@@ -308,15 +299,6 @@ class SkiaGoldClient {
       ...getCIArguments(),
     ];
 
-    if (imgtestInitCommand.contains(null)) {
-      final buf = StringBuffer()
-        ..writeln('A null argument was provided for Skia Gold tryjob init.')
-        ..writeln('Please confirm the settings of your golden file test.')
-        ..writeln('Arguments provided:');
-      imgtestInitCommand.forEach(buf.writeln);
-      throw SkiaException(buf.toString());
-    }
-
     final io.ProcessResult result = await process.run(imgtestInitCommand);
 
     if (result.exitCode != 0) {