Fix for array_keys(): Argument #1 ($array) must be of type array error

[ihabzee]

Error is caused by adding a random instrument name to the page parameter in the URL. I added a check to confirm instrument exists in the project.

The 'imagemap' module threw the following exception when calling the hook method 'redcap_every_page_top':

TypeError: array_keys(): Argument #1 ($array) must be of type array, null given in /var/www/html/redcap/modules/imagemap_v1.11.0/ExternalModule.php:55
Stack trace:
#0 /var/www/html/redcap/modules/imagemap_v1.11.0/ExternalModule.php(55): array_keys(NULL)
#1 /var/www/html/redcap/modules/imagemap_v1.11.0/ExternalModule.php(31): ImageMap\ExternalModule\ExternalModule->injectImageMaps()
#2 /var/www/html/redcap/redcap_v14.1.1/ExternalModules/classes/ExternalModules.php(3199): ImageMap\ExternalModule\ExternalModule->redcap_every_page_top('1825')
#3 /var/www/html/redcap/redcap_v14.1.1/ExternalModules/classes/ExternalModules.php(3366): ExternalModules\ExternalModules::startHook('imagemap', 'v1.11.0', Array)
#4 /var/www/html/redcap/redcap_v14.1.1/ExternalModules/classes/ExternalModules.php(3399): ExternalModules\ExternalModules::ExternalModules\{closure}('imagemap', 'v1.11.0')
#5 /var/www/html/redcap/redcap_v14.1.1/Classes/Hooks.php(41): ExternalModules\ExternalModules::callHook('every_page_top', Array)
#6 /var/www/html/redcap/redcap_v14.1.1/Classes/HtmlPage.php(150): Hooks::call('redcap_every_pa...', Array)
#7 /var/www/html/redcap/redcap_v14.1.1/Config/init_functions.php(640): HtmlPage->PrintHeader()
#8 /var/www/html/redcap/redcap_v14.1.1/Libraries/PEAR/Auth.php(587): loginFunction('dsfdsf', -3, Object(Auth))
#9 /var/www/html/redcap/redcap_v14.1.1/Libraries/PEAR/Auth.php(530): Auth->login()
#10 /var/www/html/redcap/redcap_v14.1.1/Classes/Authentication.php(1922): Auth->start()
#11 /var/www/html/redcap/redcap_v14.1.1/Classes/Authentication.php(894): Authentication::checkLogin('', 'table')
#12 /var/www/html/redcap/redcap_v14.1.1/Classes/Authentication.php(647): Authentication::tableBasedLogin()
#13 /var/www/html/redcap/redcap_v14.1.1/Classes/System.php(936): Authentication::authenticate()
#14 /var/www/html/redcap/redcap_v14.1.1/Config/init_project.php(7): System::initProjectPage()
#15 /var/www/html/redcap/redcap_v14.1.1/DataEntry/index.php(6): require_once('/var/www/html/r...')
#16 {main}