#48888)

Showing a minimum app to reproduce the bug.
1. Create the app and add angular material.
```
ng new project
cd project
ng add @angular/material
```
1. Overwrite the src/app/app.component.html with minimal content.
```
<button mat-button *ngIf="true"><span *ngIf="true" class="{{'a'}}"></span></button>
```
1. Run the app. The button is not shown because of an exception.
```
main.ts:6
ERROR TypeError: item.toLowerCase is not a function
    at isCssClassMatching (core.mjs:8726:35)
    at isNodeMatchingSelector (core.mjs:8814:22)
    at isNodeMatchingSelectorList (core.mjs:8931:13)
    at matchingProjectionSlotIndex (core.mjs:14179:13)
    at Module.ɵɵprojectionDef (core.mjs:14222:49)
    at MatButton_Template (button.mjs:113:99)
    at executeTemplate (core.mjs:10534:9)
    at renderView (core.mjs:10356:13)
    at renderComponent (core.mjs:11529:5)
    at renderChildComponents (core.mjs:10216:9)
```
Because isCssClassMatching() function does not take care if the value is not string, while attrs[] may contain AttributeMarker which is actually numbers, item.toLowerCase() throws the exception.
Just inserted a check if the item is string.

Created a testcase for the original fix. It causes an exception without the fix.

fix(core): add a check code to avoid an exception inside isCssClassMatching

Showing a minimum app to reproduce the bug.
1. Create the app and add angular material.
```
ng new project
cd project
ng add @angular/material
```
1. Add `import { MatButtonModule } from '@angular/material/button'`,
   and also MatButtonModule inside @NgModule imports in src/app/app.module.ts to use MatButtonModule.
1. Overwrite the src/app/app.component.html with minimal content.
```
<button mat-button *ngIf="true"><span *ngIf="true" class="{{'a'}}"></span></button>
```
1. Run the app. The button is not shown because of an exception.
```
main.ts:6
ERROR TypeError: item.toLowerCase is not a function
    at isCssClassMatching (core.mjs:8726:35)
    at isNodeMatchingSelector (core.mjs:8814:22)
    at isNodeMatchingSelectorList (core.mjs:8931:13)
    at matchingProjectionSlotIndex (core.mjs:14179:13)
    at Module.ɵɵprojectionDef (core.mjs:14222:49)
    at MatButton_Template (button.mjs:113:99)
    at executeTemplate (core.mjs:10534:9)
    at renderView (core.mjs:10356:13)
    at renderComponent (core.mjs:11529:5)
    at renderChildComponents (core.mjs:10216:9)
```
Because isCssClassMatching() function does not take care if the value is not string, while attrs[] may contain AttributeMarker which is actually numbers, item.toLowerCase() throws the exception.
Just inserted a check if the item is string.

PR Close #48888