Remove redundant checks for empty parameter assignments in parameter evaluation (#17807)

## Description

Remove the early return in
`EmitLimitationCalculator.CalculateParameterAssignments()` that skipped
evaluation when a .bicepparam file contained no local `param`
declarations. This ensures `extends` clauses are still resolved,
imported parameter assignments flow through, and missing/invalid
`extends` targets now surface diagnostics.

## Before change:

Logic short‑circuited if `model.Root.ParameterAssignments.IsEmpty` OR
there were parsing errors.

Example .bicepparam file with only:

```bicep
using 'main.bicep'
extends 'not-exists.bicepparam'
```

produced no parameter evaluation and (critically) suppressed the
diagnostic for the non‑existent `not-exists.bicepparam`.

Imported parameters from an extended file were silently ignored when the
extending file had zero local params.

## After change:

- `extends` declarations are always processed (unless the syntax itself
is malformed).
- Missing file now produces the expected diagnostic (e.g. BCP091).
- Parameters defined only in the extended file are now included in
evaluation/emission.
- Behavior for files that already had local params is unchanged.

## Checklist

- [x] I have read and adhere to the [contribution
guide](https://github.com/Azure/bicep/blob/main/CONTRIBUTING.md).

###### Microsoft Reviewers: [Open in
CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/Azure/bicep/pull/17807)

Author: polatengin

src/Bicep.Core/Emit/EmitLimitationCalculator.cs

@@ -609,8 +609,7 @@ private static ImmutableDictionary<ParameterAssignmentSymbol, ParameterAssignmen
             ParameterAssignmentEvaluator evaluator,
             IDiagnosticWriter diagnostics)
         {
-            if (model.Root.ParameterAssignments.IsEmpty ||
-                model.HasParsingErrors())
+            if (model.HasParsingErrors())
             {
                 return ImmutableDictionary<ParameterAssignmentSymbol, ParameterAssignmentValue>.Empty;
             }