Avoid Invalid Cast exception in EmptyMethodInspection

[BZngr]

This PR addresses the invalid cast exception reported by @daFreeMan in chat.

2019-10-16 12:03:10.8715;WARN-2.4.1.5086;Rubberduck.Inspections.Rubberduck.Inspections.Inspector;System.InvalidCastException: Unable to cast object of type 'Rubberduck.Parsing.Symbols.ExternalProcedureDeclaration' to type 'Rubberduck.Parsing.Symbols.ModuleBodyElementDeclaration'.
at Rubberduck.Inspections.Concrete.EmptyMethodInspection.<>c__DisplayClass1_0.b__0(Declaration member) in C:\projects\rubberduck\Rubberduck.CodeAnalysis\Inspections\Concrete\EmptyMethodInspection.cs:line 45
at System.Linq.Enumerable.WhereSelectEnumerableIterator2.MoveNext() at System.Linq.Enumerable.WhereEnumerableIterator1.MoveNext()
at System.Linq.Enumerable.Count[TSource](IEnumerable1 source) at Rubberduck.Inspections.Abstract.InspectionBase.GetInspectionResults(CancellationToken token) in C:\projects\rubberduck\Rubberduck.CodeAnalysis\Inspections\Abstract\InspectionBase.cs:line 112 at Rubberduck.Inspections.Rubberduck.Inspections.Inspector.RunInspection(IInspection inspection, ConcurrentBag1 allIssues, CancellationToken token) in C:\projects\rubberduck\Rubberduck.CodeAnalysis\Inspections\Inspector.cs:line 179;System.InvalidCastException: Unable to cast object of type 'Rubberduck.Parsing.Symbols.ExternalProcedureDeclaration' to type 'Rubberduck.Parsing.Symbols.ModuleBodyElementDeclaration'.
at Rubberduck.Inspections.Concrete.EmptyMethodInspection.<>c__DisplayClass1_0.b__0(Declaration member) in C:\projects\rubberduck\Rubberduck.CodeAnalysis\Inspections\Concrete\EmptyMethodInspection.cs:line 45
at System.Linq.Enumerable.WhereSelectEnumerableIterator2.MoveNext() at System.Linq.Enumerable.WhereEnumerableIterator1.MoveNext()
at System.Linq.Enumerable.Count[TSource](IEnumerable1 source) at Rubberduck.Inspections.Abstract.InspectionBase.GetInspectionResults(CancellationToken token) in C:\projects\rubberduck\Rubberduck.CodeAnalysis\Inspections\Abstract\InspectionBase.cs:line 112 at Rubberduck.Inspections.Rubberduck.Inspections.Inspector.RunInspection(IInspection inspection, ConcurrentBag1 allIssues, CancellationToken token) in C:\projects\rubberduck\Rubberduck.CodeAnalysis\Inspections\Inspector.cs:line 179

[MDoerner]

LGTM

External procedures are considered to be members since they are used like functions. However, they do not have a body in contrast to module body elements and thus are not in scope for this inspection.